I know this is an old issue, but I recently found my dad’s old rebel XTI and bought new batteries to mess around with it for the nostalgia. Everything was functioning perfectly until all of a sudden a few days later, I was experiencing the same flash issue as you.
Turns out I placed a hot shoe cover on top of the camera and this prohibits the flash from charging and firing even if popped up forcefully.
As soon as I removed it, the flash fired as normal.
Came across this thread in search of the solution to my problem so hope this helps someone.
