Sounds more like a pressure problem to me. If the sprinkler head is throwing a tighter stream of water more towards the outer reach of its radius that is what is called "a donut" effect. This is typical of low pressure at the head.May I suggest ,first check and insure you have the right size nozzle in each head and maybe switch to a smaller one. Secondly, try eliminating one of the heads and see how it operates then. You did not mention what your static water pressure was and what size the water supply line is.