First, let me warn anyone new to the book biz: nothing is set in stone. Aside from the Ten Commandments, that is.
Embedded in a writer's mind is the idea that once the tough struggle of finding a house to publish your life's work with, all the hard times are over. I know for a fact that's never been the case. As publishing got more and more competitive, the big houses kept upping the ante on authors, demanding higher sales and dumping them from contracts if they fell behind. In the late 80's many of my mid-list friends found themselves in limbo with a series in place but now homeless.
The ground response was three-fold: small publishing outfits popped up, Publish America and I Universe took hold and authors created their own publishing houses to get their works out there. Many ventures failed and, well, we pretty much know what happened to PA and IU. Nada.
I think the best tactic these days is having a contingency plan. Assume nothing good will last forever. I advise against authors holding on to their manuscripts waiting for that “sure thing.” In this industry, there is no sure thing. The smartest move a writer can make is to get their work out there as soon as possible. All this rewriting toward perfection—newsflash: perfection is an illusion. What does exist is a competition that won't wait. My mantra is “You can't market what doesn't exist.” You don't exist until you have something out there to market to readers. Understand that this industry moves at breakneck speed and use that to your advantage. Nobody is going to ding you for a mediocre book. Create a nom de plume for your next work.
Here's three reasons you may need to search for a new publisher:
Scenario #1: Money issues. Unless the publisher is independently rich, there are always going to be money issues. Love of the written word does not put food on the table or pay the rent. Many small houses operate from actual small houses. This is possible with today's technology and the publisher's ability to handle a computer. Publish on Demand (POD) and email have changed the industry. No more costly offices and staff or warehouses for book stock. But, this also means the publisher can fold unexpectedly at any time.
Scenario #2: Health. When a small publishing outfit is run by one or two people, health can be an issue. If the publisher gets sick, books go unpublished. If the publisher dies, the house ceases to exist. So do contracts.
Scenario #3: Personal conflict. Sometimes you aren't going to see eye-to-eye with a publisher. Maybe it's over the delay in royalties. Possibly the quality of your latest novel. Or, you just get on each other's nerves.
So, how do you find a new publishing house?
First, brush off all your contacts. You should have been storing them up for just this moment. You know others who are published, find out about their house. Ask for a recommendation. Ask nicely.
Second, scout around for small houses you've never heard of. Don't bother with Writers' Digest because once they list a house, everyone dashes over there. Keep an eye out for outfits just starting up and hungry to build a stable. They're all over the Internet. Try Predators and Editors. You've published before, you have a track record. You may be the big fish they're looking for.
Third, go on Amazon and find a novel close to what you publish. See who the publisher is. I'm not talking about bestsellers and, if that's what you think you've written, you aren't looking for a new home. No, explore the book catalog and see how many books in your genre are listed.
Fourth, cold call (email) some of the authors and ask (nicely) if they are happy with their publisher. Don't do this on group lists, very bad form and you'll get some nasty responses. Most authors will be honest with their replies. If they hedge, take a pass.
Fifth, roll the dice. You've done your homework but, like I said in the opening paragraph, nothing is set in stone.