We replaced our timer 2 years ago and a couple of valves about 4 years ago. We have 6 valves connected to the timer. They all worked good until recently. The first valve to go on, only stays on about 30 seconds and turns off. It is suppose to stay on for 5 minutes. The timer is still going and after 5 minutes, the next station goes on and the rest run like they should. What can be causing this?