Sapper.
This may be a stupid question but are you rotating your foot correctly when trying to exit the pedals?
I had a mate that got spds for the first time and kept falling off as he could get his feet out of the pedals.
It took my a while to figure out why until I asked him to show me how he was getting in and out of the pedals.
It turns out he was trying to twist his foot out by rotating the ball of his foot outwards as opposed to rotating his heal outwards, since nobody had actually told him this I guess it was an easy mistake to make.