Is the soleniod on tight (not too tight, just hand tight) if not, maybe tighten that. The pipe might be for drainage, what kind of soil do you have? The increase the flow on the line that you added more heads to, maybe put a small nozzle within your GPM and PSI. It might make a LITTLE difference, but not much. You might have to remove the heads that you added and cap the risers or cut the pipe that you added for the new heads off. If you added more heads because of dry spots, I would hand water those areas if there isn't too many of them.
Hope this helps