The Muaka and Kane-ra set I have, for some reason Muaka's jaw doesn't fully open like Kane-Ra's does. This is unless I move the jaw to realign the 6641 piece (https://www.bricklink.com/v2/catalog/catalogitem.page?P=6641&idColor=9#T=C&C=9) with the cog EVERY time I use the function. Are either of those pieces the problem or is it something else?
Do you have a picture of the build.
Well I imagine it's as usual just with 6641 to the side of the cog part it should go under.
Could be an error in the way it is constructed or maybe it has to do with old rubber bands.My advice either you rebuild Muaka or you change rubber bands
Might need to rebuild it