I don't know if this is an ongoing issue. My recollection is that I just had to recreate the entire Flow and try again. I don't believe that I ever understood what was wrong with the Flow that would not register, so I cannot try to reproduce to test.
I have just experienced this problem again. I have a quite complex Flow for an approval process that was all working and now it just won't start from my PowerApp. It started coming up with a runtime error "trigger is not enabled" which I have not been able to find any information about.
I then removed the Flow from my app and now I have the dreaded "Failed to Register Service" error when I try to register the Flow again.
Please advise what steps I can take to try to fix this? I see other posts here talking about the problem being with the auto generated JSON schema. Can anyone help me with looking at this?