I assume you have located and are pressing the locking tab that holds the two parts together. If so, there can still be considerable resistance because this type of connector has a rubber gasket inside that seals out water and crud.
Make sure you have the tab fully unlocked and you are grasping the two parts, one in each hand, and pull firmly. Do not pull on only one side of you may tear the wires or pull the connector pins out the back.