Why does the iPad lock up and not accept a correct passcode?

I've been using Apple products for years. But lately, I've been frustrated with one of those products in particular. The iPad. Why do iPads lock up and not accept your passcode? The prompt on support assumes that you're using the wrong passcode, but I am not. I don't use my iPad often, but when I do, I don't want to go through lots of hassle to get it up and running. In order to use it today, I've had to plug it into my Mac and reset it. However, even the resets don't work. Arrrgh!