It can seem, sometimes, that agents make too little effort on behalf of their clients—particularly their newer, unpublished ones.
There are relatively few major fiction publishers in the United Kingdom. If an agent can’t place a book after ten or fifteen submissions, they might not try further: chances are there won’t be any other good-enough, big enough publishers which they think would “fit” the book well. So it might seem that an agent has made only a handful of submissions before advising their disappointed clients that the book has failed to sell, and that they should move on and write a new, more commercial book. In this situation writers are often left feeling that there’s not been nearly enough effort made on behalf of their precious books—especially when they consider the pages and pages of publishers that are listed in the Writer’s Handbook. To make it worse, it’s almost impossible for a writer to then find another agent to take their book on, as the highest-earning routes for it have already been exhausted.
But all is not lost. There are some wonderful smaller presses which fall beneath most agents’ radars because of their lower (or non-existent) advances. These presses usually accept unagented submissions, so there's nothing to stop a writer from making submissions for themselves if their agent gives up on their books.
Some of these presses produce beautiful books, and achieve sales that the bigger presses envy: Sarah Bower's The Needle in the Blood, from the consistently good Snowbooks, and Catherine O'Flynn's What Was Lost (or try this edition, with a much-improved jacket design), from Tindal Street Press, are prime examples.
Writers have to be careful: there are plenty of vanity publishers out there masquerading as small presses. And I'd still always advise a writer to find an agent to check and negotiate any contract before they signed. But if an agent is unable to place a book, that doesn't mean it's unpublishable.