Hi James,
Regarding the robots.txt syntax:
Disallow: /jobs/? which basically blocks every single URL that contains /jobs/**? **
For example: domain.com**/jobs/?**sort-by=... will be blocked
If you want to disallow query parameters from URL, the correct implementation would be Disallow: /jobs/*? or even specify which query parameter you want to block. For example Disallow: /jobs/*?page=
My question to you, if these jobs are linked from any other page and/or sitemap? Or only from the listing page, which has it's pagination, sorting, etc. is blocked by robots.txt? If they are not linked, it could be a simple case of orphan pages, where basically the crawler cannot access the job posting pages, because there is no actual link to it. I know it is an old rule, but it is still true: Crawl > Index > Rank.
BTW. I don't know why you would block your pagination. There are other optimal implementations.
And there is always the scenario, that was already described by Matt. But I believe in that case you would have at least some of the pages indexed even if they are not going to get ranked well.
Also, make sure other technical implementations are not stopping your job posting pages from being indexed.