Well many people have failed TVRs, they are harder to get these days because well face it, they are trying to avoid people who are not going to leave the country. And whether you can prove you are a real visitor or not, I think alot if not most will be rejected during at least one attempt (my spouse included...)
A complicated case for example but not limited to...both divorced, or in process of a divorce, where and how you met, how long your courtship has been, are there children involved, amount of proof you have for your relationship etc etc. Like I said list is not exhaustive and typically one of these things is not such a big deal but in combination with one or more... yeah VOs start taking a good look. Typical mistakes that people make and coudl alleviate the problems... putting ample amount of proof of relationship (pictures, texts, phone bills , cards, e-mails , letters of support from family and friends etc etc.)
Other posters can probably offer you some more advice... but honestly if you are feeling unsure, find a consultant.