Hi. There seems to be a fault with the Tab 8. If a security pin is set up then when the device locked, it doesn't accept the pin and stays locked. The only fix is a factory reset.
It works fine with a security pattern, but not a pin.
I see other people are having the same trouble too - https://uk.trustpilot.com/reviews/5f3e609802e85708c8d4ef05
thanks, Gordon