Spray heads don't come up.....

I have one zone that doesn't seem to have enough pressure to push the spray heads up. All of the other zones work fine. When the faulty zone is activated, water comes out of each head, but weakly. My first thought is that the valve may need to be repaired or replaced. Not having too much expertise in irrigation matters, I can use some guidance to troubleshoot the problem and fix it........please respond ASAP.....thanks, Buz

Spray heads don't come up.....