I've have had occassional problems with setting MWB at depth. What I discovered is if the WB button is left depressed, hitting the select button does nothing. I've been able to duplicate this both in and out of the housing. Something similar might be happening with the gain/shutter.
Solution is to turn the knob out away from the cam once you've selected the mode you want to be in. Then the roller/select button works fine.
