Wet mortar shells

[Mahatatain]

I recently watched the series "The Pacific" and there is a scene in it where the mortar shells get wet and therefore can't be used, rendering the mortar team useless until they get some dry shells. Is this detail of mortars accurate and is it still an issue that could be used for flavour in a T2k or T2k13 game (i.e. it's not just a WWII issue)?

Also what is the problem with wet mortar shells?