While I do agree that there are difficulties in explaining the value to clients, the most common reasons I have come up across are accounting & cash flow.
When I was managing agency side and I just wanted an additonal $1000/mo (which seems like nothing for most advertisers), there was significant push back. One reason is the client is not only footing the bill for their account, they are also footing the agency bill. There could be added costs when expanding budgets depending on the pricing structure, and everyone wants everything as cheap as they can get it. So if the added cost of the agency + the added cost on the business for the advertising is too much in comparison to the budget the business had put forward for the year, then there is no way they can plan on spending more.
In addition to this, if you are working with smaller businesses, they don't have the flexibility in their cash flow to spend more even when there is a proven opportunity. Not to mention, advertising is very likely not the only bill they are paying each month. They have all the costs associated from man power to tools & software internally to operational overhead. They might say their profit is $65/item but it might actually be more like $15 after everything else is accounted for. Which is poor planning and communication from your client.
If you are working with a larger or public company, accounting requires that you bring in X amount of money (because Stock Market or Parent Company, or Investors, etc). If you are advertising based on LTV (lifetime value) not New Sales, then the accounting team doesn't see the same return you are assuming you are getting. It appears to the accounting team that you are overspending and not getting the same value on more cost compared to previous years.
But, take your client out for a beer if you can. Get them to unload on you about the issues and tribulations that they are coming up across internally or if the company does have a cash flow issue. There are real business reasons behind not spending more, and it might be as minor as "we are exploring another channel that is proving to be quite costly right now" and wont' have anything to do with you or your work.
Good luck! This is the hard part of consulting