Most likely batteries are low - so Id try that first.

Other than that I've found contacts on the buttons in toys can be fairly simple design - with just two metals being touched together - and they get tarnished or wet and dont make contact as cleanly as when new. disassembly and cleaning to shiny contacts usually fixes it.
Dont use anything too abrasave - i find even photo copy paper rough enough to polish the switch contacts to like new condition.