Could be low pressure. Check the "Kicker Arm" to make sure it's clear of debris. Push down on the head and turn the head to the left and right quickly - possible grit or sand in the seals. Check the spray nozzle to see if something is lodged in the opening. This doesn't work, take the piece of crap back to the store and get a Rainbird 15103-09 Impulse Head. They are the best impulse heads ever made.
Richard Kersten
S&R Sprinkler Systems and Landscaping, Inc.
Destin, Florida