My "three" and "four" keys sometimes work, and sometimes dont

A couple of weeks ago, my three and four keys suddenly stopped working. sometimes, they will work if I press them repeatedly, and sometimes they dont work at all, and i have to copy the number from somewhere else. I tried pulling off the key caps and couldn't see anything wrong, but I'm not entirely sure what i should be looking for. is there a way to fix this without buying a whole new keyboard?