"James" wrote in message
. ..
I would think that if there were a clog in a sprinkler head, why would they
work part of the time, but will not work at other times ?

When the water kicks on, the rocks (obstructions) go to the orifice in the
sprayer. Another time, they don't, or rearrange themselves differently.
Each time, they drop to the bottom waiting for the next wave. And every
time, it's different until they become clogged in one spot.

Steve