FOF: Why don't the PR/KR's always take kicks?
 

If you set them to take kick returns and punt returns and even lock them, why do random flankers and split ends sometimes take kicks?

I'm guessing for two reasons:

1. Short kicks taken by the up-back.
2. PR/KR's who play on offense or defense, and are tired at the time of the kick, and therefore are replaced by the AI's choice.

...and locking the players has no effect on PR/KRs when playing a game, it matters only when you hit the "Recommend" button on the depth chart screens.

and if you have coach doing depth charts in the global options , it wont matter what you do on the depth chart...

My guess would be like the real NFL, maybe the other team kicks away from them, short and stuff.