Spraying with silicone lube might work but silicone lube is sticky so they will get dirty fast. Flash of follower isnt likely the problem as it would be constant on all rounds. A slight burr or dent or ding in the mag at a certain location that happens to be where Round 6 feeds is more likely. I would definitely take it apart, clean and inspect. Never a bad thing.
Mags are not that expensive, you could just buy another one.