I share your frustration. I replaced both of our coach's drain valves several years ago and have continued having a problem with opening them. So far, it has worked for me to work the electric rocker switch back and forth between OPEN and CLOSE while holding the switch in each position for no more than a second or so after hearing the sound of the moving valve. The lady at Drain Master warned me to not hold the switch in either the OPEN or CLOSE position for longer than about two seconds. Otherwise, you run the risk of overheating and burning out the valve motor. As I said, so far by pressing the electric switch back and forth, sometimes as many as ten or more times, I have been successful in getting the valves to open and close. Once I get the valves to move, it seems they will operate properly if again used within a few days or a week.
Another trick I use is to put about a quart of cheap cooking oil into both tanks to lubricate the valves when the coach will be sitting for weeks or months without being used.
As you pointed out, overtightening the four bolts on the valves will make the motor have to work harder to open/close the valves. I know I overtightened mine just a bit when installing them.