I only ever got about 3 years out of one, it seemed to me that age got them more than use. As was said, check your battery voltage, make sure you have a good solid ground. I have clamped a vise grip to hold them open(after removing the magnet) and finished a day of spraying but turning the pump on and off.

Then, next winter, take them off and throw them wherever you go with the rest of your scrap and get ball valves. DO NOT replace them with more solenoids, put that money towards ball valves.