How can I remove a pool strainer cover that is on too tight?

I cannot remove the pool strainer plastic cover to get to the strainer basket. I think that it is the suction type of cover as there is a 1/4 gate valve on it-I have no idea what I'm doing, so I don't want to break anything. It's almost like the suction is to!o strong to let go of it. If anyone can help me I'd truly appreciate it