My falsetto tends to randomly go away and come back. I've tried a lot of things like trying to place the sound in different places but none of them work consistently. It's really hard to explain this but I will try to as best as I can. I will be singing the chorus of a song and the first 2 times I go through it the falsetto notes will be fine, but then the 3rd time I go through it and using the exact same vocal technique all that will come out is air. It stays like this for around 5 minutes usually and then it works again. It's really confusing and some days I lose my falsetto completely.
If you could record when your falsetto fails, I might be able to help you further.