That is normal. With time and heat, a rubber gasket will stick to the mating surfaces. Personally I do not like prying them up, to avoid scratches or bending something. Usually a couple light taps with a rubber mallet will do it.

If one is not on hand, I would lay a piece of wood against the valve cover as a cushion, and then tap the wood with a hammer. Once the seal is broken, it should come right up.