The solenoid is what's giving you problems. They are electro-magnetic coils that when energized, pull a small plunger up and uncover the bypass to activate the valve. Most everyone I've ever seen takes 24 volts to energize (actually the transformers that operate them produce 27.9 to 28 volts AC). Any hardware stroe that carries irrigation equipment should have a plug in transformer that should remedy your problem. Check the instructions (or check on the solenoid) to see if this is what you need, but it really sounds like you don't have enough voltage to activate the solenoid on a regular basis.