This is probably more of a ranking authority problem, rather than an indexation problem. If you can force Google to render one of your category URLs within its search results, then it's highly likely the page is indeed indexed (it's just not ranking very well for associated keywords)
Follow this link:
https://www.google.co.uk/search?q=site%3Askirtinguk.com%2Fproduct-category%2Fmdf-skirting-board%2F
As you can see, the category URL which you referenced is indexed. Google can render it within their search results!
Although Google know the page exists and it is in their index, they don't bother to keep a cache of the URL: http://webcache.googleusercontent.com/search?q=cache:https%3A%2F%2Fwww.skirtinguk.com%2Fproduct-category%2Fmdf-skirting-board%2F
This probably means that they don't think many people use the page or that it is of low value.
What you have to keep in mind is, lower value long-tail terms (like product keywords or part number keywords) are much easier to achieve. Category terms are worth more in terms of search volume, so competition for them is higher. If your site ranks for product terms but not for category terms, it probably means your authority and / or trust metrics (as well as UX metrics) may be lower. Remember: Google don't consider their ranking results to be a space to advertise lots of companies. They want to render the best results possible for the end-user (that way people keep 'Googling' and Google continue to leverage revenue from Google AdWords etc)
Let's look at your site's domain-level metrics and see if they paint a picture of an 'authoritative' site which should be ranking for such terms...
Domain Level Metrics from Moz
Domain Authority: 24 (low)
Total Inbound Links: 1,200+
Total Referring Domains (much more important than total link count!): 123 - This is too many links from too few domains IMO
Ranking keywords: 38
Domain Level Metrics from Ahrefs
Homepage URL Rating: 11 (very low)
Domain Rating: 11 (very low)
Total Inbound Links: 2,110+
Referring Domains: 149 - Again, the disparity here could be causing problems! Not a diverse backlink profile
Ranking Keywords: 374 (Ahrefs usually finds more, go with this figure)
SEO Traffic Insights: Between 250 and 380 visits (from SEO) a day on average, not much traffic at all from SEO before November 2016 when things improved significantly
SEMRush Traffic Insights (to compare against Ahrefs): Estimates between 100 and 150 visits from SEO per day. This is narrowed to UK only though. Seems to tally with what Ahrefs is saying, the Ahrefs data is probably more accurate
Domain Level Metrics from Majestic SEO
Trust Flow: 5 - This is extremely low and really bad! Basically Majestic track the number of clicks from a seed set of trusted sites, to your site. A low number (it's on a scale of 0 to 100 I think) indicates that trustworthy seed sites aren't linking to you, or that where you are linked - people avoid clicking a link to your site (or visiting it)
Citation Flow: 24 - low but now awful
What do I get from all of this info?
I don't think your site is doing enough digital PR, or making 'enough of a difference to the web' to rank highly for category related terms. Certainly the site looks very drab and 'cookie-cutter' in terms of the template. It doesn't instil a sense of pride in the business behind the website. That can put people off linking to you, which can cause your SEO authority to fall flat on its face leaving you with no ranking power.
A lot of the product images look as if they are fake which probably isn't helping. They actually look at lot like ads which often look a bit cartoony or CGI-generated, with a balance between blue and white (colour deployment). Maybe they're being misinterpreted as spam due to Google PLA (Page Layout Algorithm). Design is not helping you out at all I am afraid!
So who is ranking for MDF skirting board? The top non-PPC (ad-based) result on Google.co.uk is this one:
https://skirtingboardsdirect.com/products/category/mdf-skirting-boards/
Ok so their content is better and deeper than yours (bullet-pointed specs or stats often imply 'granular' content to Google, which Google really likes - your content is just one solid paragraph). Overall though, I'd actually say their design is awful! It's worse than the design of your site (so maybe design isn't such a big factor here after all).
Let's compare some top-line SEO authority metrics on your site against those earned by this competitor
skirtinguk.com
- Domain Authority from Moz: 24
- Referring Domains from Moz: 123
- Ahrefs Homepage URL Rating: 11
- Ahrefs Domain Rating: 11
- Ahrefs Referring Domains: 149
- Majestic SEO Trust Flow: 5
- Majestic SEO Citation Flow: 24
Now the other site...
skirtingboardsdirect.com
- Domain Authority from Moz: 33 (+9)
- Referring Domains from Moz: 464 (+341)
- Ahrefs Homepage URL Rating: 31 (+20)
- Ahrefs Domain Rating: 65 (+54)
- Ahrefs Referring Domains: 265 (+116)
- Majestic SEO Trust Flow: 29 (+24)
- Majestic SEO Citation Flow: 30 (+6)
They beat you in all the important areas! That's not good.
Your category-level URLs aren't Meta no indexed, or blocked in the robots.txt file. Since we have found evidence that Google are in fact indexing your category level URLs, it's actually a ranking / authority problem, cleverly disguised as an indexation issue (I can see why you assumed that). These pages aren't **good enough **to be frequently indexed by Google, for keywords which they know hold lucrative financial value. Only the better sites (or the more authoritative ones) will rank there
A main competitor has similar design standards but has slightly deeper content and much more SEO authority than you do. The same is probably true for other competing sites. In SEO, you have to fight to maintain your positions. Sitting back is equivalent to begging your competitors to steal all of your traffic...
Hope this analysis helps!