iPad Doesn’t Hold Passcode

I bot iPad10 2 months ago. I know the passcode but apparently the iPad doesn’t hold it. If I don’t use the device for a few days, it prompts me to enter the passcode but doesn’t accept it as valid. I’ve had to reset the iPad multiple times in order to get in, and I’ve gone so far as to write the passcode so I’m sure to use the correct one, but the next time I’m prompted for it, it fails & makes me wait to try again. I’m tired of constantly resetting. Why is this happening?