Make sure the bullet clears the slide stop nub:

Make sure the back edge of the slide stop isn't rounded and that the slide itself isn't rounded where the two mate to hold the slide open (under the R on my Rami):

It's a feature if it drops the slide after seating the magazine but if it happens before seated that's a no go for sure!
It might also be time to replace magazine springs (or pull apart and clean and wax them).
Cheers,
Toby
Sent from my SM-G950U using Tapatalk