This issue should be fixed at the heads, via orifice selection, and not by stepping down the size of the feed pipe. Typical average-household-income-sized residential sprinkler systems run a 3/4" or 1" main and 3/4" laterals to ALL heads.That is your problem. How long is the run? You likely have too large of pipe but for sure you need to size down approx half way down the run. If you were to start over, and not saying you should, you would use 3/4" then go down to 1/2" about half way on the length of the run. If you have T's off the main line then you would use 1/2" there also. You will see a significant difference than what you have now.
Wrooster