I came across this link in my quest for interesting writing websites. Carina Press is a site set up for a Digital only publishing arm of Harlequin Press one of the largest romance publishers in the world.
Like all Harlequin sites, there's a wealth of information there including in the above link a helpful list of reasons they reject manuscripts. If you've had submissions sent back to you without any kind of hint as to why then this list will be helpful.
Deal with all their suggestions and you'll have a winning book.