The locks become truly "locked" over time. This makes it hard for the spring compressor to release the half locks. So...
Before you remove the valves after they have been run, place a large socket under the valve head you are working on, so the head rests on the socket. Then take another deep socket, about 9/16 to 7/8", and place it over the valve spring retainer. Rap it hard with a hammer. That will break the tension on the half locks so that the spring compressor can work easily. It is possible the locks will fly out when you do this, if you hit too hard...thus the deep socket to keep everything together! But, you only need to hit them hard enough to feel the spring flex. That means the locks are loosened.