There are several reasons a romance novel fails, and that always has to do with the development of the romance between the two love interests.
Every character in a story is going to have a main goal and something they want or need.
In romance novels, this ends up being the relationship that ends in a happily ever after. Most of the time. That darn Nicholas Sparks!
Obviously, romance sub-genres will have plot layers if mystery or suspense is involved, but when we're talking straight romance, it's far more noticeable if the development of the relationship is off.
Here are just a few issues I've seen in romance novels.
1.) Love in a vacuum: nothing is happening other than the characters suddenly falling in love for no discernible reason.
2.) Purely Physical: the romantic tension relies on nothing more than the physical aspects of the relationship, preventing any development of emotional attachments taking place. (Note: erotica is a whole other subject, and its readership generally expects there to be explicit content when forming the relationship. Nothing wrong with that if that's the kind of readership you are aiming at. Just know your audience and write accordingly.)
3.) Little or no romantic tension: romantic tension involves wanting without fulfillment. Even the anticipation of a simple kiss can cause enormous amounts of tension if the two love interests are always close to succumbing but never quite get there.
4.) Weak sources of conflict: Are the obstacles that prevent the love interests from coming together superficial and unbelievable? Consider what each character wants and what they need, and put those wants and needs in opposition to one another, causing their relationship to seemed doomed before it even starts.
5. The reader doesn't care: are your characters likable, easy to relate to, or people we can admire? If they don't have any redeeming qualities, interesting backstories, and quite possibly some deep, dark secret that prevents them from moving on and falling in love, then you might end up with characters who fail to inspire readers or pique their interest.
There are three things every romance needs.
!.) An emotional connection between characters: they can't just be really attractive. Lust does not build a connection. Emotional connections require interaction and time. There also needs to be an emotional connection for the reader. They need to see the strengths and weaknesses of your character. No one can relate to a perfect person. There needs to be a compelling reason for your couple to be together. This helps with avoiding: the love vacuum, reader caring, cliche characters and plot.
2.) Need fulfillment: what does the character need? The deeper the need the deeper the connection. So figure out what your character needs in a significant other. This helps with: the love vacuum, weak source of conflict, cliche character, and no foundation for the love.
3.) Unique connection: the couple is something to each other that no one else is or can be. If the connection isn't unique, it will lack impact and will not be satisfying to your readers. This is why their connection must go beyond love at first sight, infatuation, or physical pleasure. This helps with poor reasons for miscommunication or no communication, the love has no foundation, little or no romantic tension.
I'm always happy to answer questions on the fundamentals of fiction writing if you would like to discuss romance or any other genre in greater detail.