Sprinkler heads not popping up
Sounds like a broken line under ground between the point you are checking the pressure and the Heads, with the pressure at 20 there is probably not enough to push the head up but it should be flowing even though not extended.

When I have this problem I will usually leave the water on for a while and go find the wet spot. Dig down and fix the break. If the pipes were full and you had a cold winter there may be several broken lines. 

Also running the water to find the wet spot always gets some dirt and possibly gravel in the line so I will remove the heads and flow all the dirt out , or cleaning the heads then flowing the stuff out then putting the heads back on. Best to pull them first.