Need opinioins on a problem with wiring/ valves.

I have an odd problem. I have 7 zones in a new system. 4 are in the front yard and are on one buried bundle of 16 gauge wire, attached to a basement rainbird controller. The 3 zones in the back yard are on a separate bundled cable of wires. Only one of the zones in back has sprinklers, other 2 are drip. All are attached to a rainbired 9 zone STPi , easiest controller I have ever seen to set up.

Front zones function flawlessly, identical function with manual or electronic turn on. Sprinkler zone in back works fine manual (1/4 turn on the solenoid) but works @ about 40-50% of capacity when turned on electronically.

I tried changing solenoid-No change. Electrician and local watering supply suggested voltage drop due to the length of wire to the box (>200 ft) Before I tried to bury a second wire in a better position, I took the controller off the wall in the basement and put it in the back, connected the valve to the controller with SUGGESTIONS or EXPLANATONS welcome.

Thanks ons about a valve problem: