A shooter today was firing a .45acp and every third round, the slide would stop returning to battery with a the bullet nose stuck on the feed ramp and the case still half way in the magazine ??
Limp Wristing ??
Limp Wristing ??
In addition to what armorer951 stated, I would also suspect limp wristing first. You don't mention the firearm type or the shooters ability.
Why don't you test fire it for him , eliminating the possibility of "limp wristing" and see if it malfunctions for you ... at least it would easily eliminate one possible cause .
Gary