+/- Keys Sometimes Don't Work in Develop Module

Does anyone know what the problem is with this: Sometimes, probably at least half the time, when editing photos I click on a word like Exposure or any others to use the + and – keys, nothing happens and the word and number will slowly flash lighter and darker once or twice, then nothing. The only way I can get it to work again is to click somewhere else, like on the photo then click on the word again, then it works again. It doesn't even seem like it's anything I'm doing wrong, because it does work sometimes. This takes up a lot of extra time with it happening often and having to pay close attention to if it’s even working or not on each photo. The pop-up label does come up on the screen saying modify with + / - keys so it does know that I clicked on it, it just doesn’t work.