If you have checked it electrically and everything is good in that respect, I would ,as suggested prior look at the valve body. Since you say it works when you back off the solenoid or open the bleeder screw then I would certainly look at the water exhaust port which is part of the valve body. It is a small port hole in the solenoid area that allows the water to evacuate from the upper diaphragm chamber and exit the outlet side if the valve. This then allows the valve to open. If this hole is partially or fully blocked the valve will not open. You can check it by removing the solenoid and locating this small hole which generally leads from the solenoid location to the downstream side of the valve. You may insert a small piece of wire into the passageway to make sure it is clear. You can use a small piece of 18 guage irrigation wire with the casing striped off to do this.The wire should pass through very easily. Generally if you have a blockage , this wire will dislodge it and your back in business.