Your problem is probably that your valve is internally piloted. Larger valves use air pressure to shift their spool from a smaller built-in direct solenoid valve. You need to contact the manufacturer or vendor to find out how to change your valve from an internally piloted valve to an externally piloted valve. I just recently went through this with Parker valves…their two position valve requires a minimum of 25 psi to shift the spool and their 3 position valved requires a minimum of 35 psi. This is necessary to overcome the seal friction of the o-rings on the spool.