I have on section of my sprinkler system that will only raise the 5 popup sprinklers on that section about halfway, and with only a little flow at each head. If I push down and 'seal' off two heads at one time, the others rise up all the way and then flow at full volume. I then can release the other 2 heads and they stay up and sprinkler normally as well. Unfortunately this does not help when using that section on a timer, as I have thus far been unable to train the squirrels and rabbits out there to sit on two heads and wait for the pressure to rise... what's the most likely problem? FYI, that section contains the lowest sprinkler head of the whole lawn, if that makes any difference.