Ahh, I guess I know the reason:)
How many hidden stings do you have? You might be working with some specific file format that requires using duplicates as hidden (for example titles from websites), or just keep some strings/files hidden for some reasons.
And by default only Owner and Manager can access them, and as a result do some actions like replacement. With some project settings change about proofreaders you can increase their rights and they would also be able to do so (don’t remember exactly how does it called, should be something about hidden stings and proofreaders, almost in the start of Project Settings list).
I guess you can give them a try.