Can't stay wet

Okay so, me and my fiance have sex, a lot, and not always is this a problem, but, sometimes in the middle of having sex, I will just randomly stop being wet and then it just makes sex very uncomfortable, I mean like stopping to spit on it or use lube... Just kills the mood ya know!? Why is this happening? Am I broken or just not getting/staying turned on? How do I stay wet longer?