There could be a number of reasons why the Water Main Shut-Off isn't closing or opening your valve completely. Review the list below and try the suggested tweaks. If they fail to help, please contact our support staff!
Sediment Build-Up/Aged Valve
Over time, most valves might become difficult to use because of loss of lubrication or build-up of sediments inside the valve. To work past this limitation, first make sure the valve is operable by hand. Open it and close it multiple times prior to installation to make sure it's completely loose. Then, apply WD40 (which will dissolve old "caked on" lubricants and grime) and lubricate the joint from the exterior. This should allow the valve to turn easier.
Proper Axle Alignment
See Figure 1, below. The axle of the ball valve and the Water Main Shut-Off need to be completely aligned. Even a small misalignment can drastically reduce its effectiveness.
Figure 1 - Axle Alignment
Proper Control Arm Alignment
When tightening the Control Arm around the valve handle, the two "jaws" of the clamp can be shifted from one side to the other. See Figure 2, below. Notice how the Control Arm (inside the red rectangle) can be shifted side to side. Depending on which way your unit is having problems completing the motion, adjusting these jaws in one direction or the other could help with turning the valve completely.
Figure 2 - Control Arm Alignment
A final possibility is that the screws securing the Water Main Shut-Off are not tightened all the way. Inspect both hose clamps along with the four screws that secure the control arm around the valve handle, and make sure that all six of these fasteners are as tight as possible.