I have a similar problem but I don't think it's the module... maybe somebody can chime in?
The unlock doors and unlock trunk buttons work fine on my key, but SOMETIMES the lock button won't do anything and I have to manually lock the doors. The doors lock and unlock everytime so it's not as big as the OP's issue, but it pisses me off that it occasionally won't work. I went to the dealer and was originally told I need a new key at $280 a pop, no thank you I'll manually lock it for the rest of my life. When the service rep finally came out to see the car and test it, it worked every time. This has happened on 2 occasions, the service rep can lock it, but as soon as I drive off the lot I cannot.
Is it the key? The module? The temperature...? (I'm saying this because it doesn't want to lock when it's very cold out, or at least the key is ice cold).