[gatesphere]

It sounds stupid, but scotch tape on the lock switch really helps.

It was my 'fix' when one of my readers had a broken lock detector arm, and read all cards as locked even if they weren't. The scotch tape on the card fixed it :)

[ptaipale]

It does not sound stupid. It's the standard thing to do.

Here you really need to have a thin tape, Scotch cellotape is fine. In other places, the good old duct tape saves you. (We actually call it "Jesus tape" here, because it saves people.)