Crow hunting question?

We've been having some pesky crow problems in our field and a friend and I have been trying to get some of them, but have been unsuccessful. We've been going out right before dark, setting up crow decoys, and playing a dying crow call sound out of our foxpro call. We usually set up on the edge of the field in camoflauge with our backs against trees. We've had many of them come in to the call, but I think they are spotting us and leaving. I have a ground blind, but should I use it? Any tips and help would be greatly appreciated as I am very new to this. Thanks!