Setting up my Flash after a long time now that my basement is ready to have my 80's (yes, it's from 1979...) row "installed", and I had this issue a long time ago as well.
My backglass won't fit in the head correctly. It did at one point in time. I slide it in the top like any machine and it's all good so far, then when I go to let it drop in the crevice it doesn't want to go down all of the way, and it almost doesn't even to seem to go down at all. I can't even force it in by pushing down on it hard. If you're not able to get the mental image, think of it as only half of the displays showing because it is sitting too high.
I made sure the head was unlocked (then unbolted the lock mechanism and pushed it back into the head to make double sure), and both the top and bottom "line" where it goes in are now both fully clear of any debris or small objects.
I adjusted to the two metal pieces the door latches onto since you can thankfully unscrew them and slide them back so the door will close more (or less, depending on what you want). I slid them back all of the way and successfully latched the door but STILL the glass will not drop in.
At a loss here. Has anybody ever experienced this before? I must have tried 20 times. This glass already isn't a prime example but I'm hoping to finally let it go in so I don't damage it if I haven't already.