It's very simple. These support guys on support waypoints are bastards.
I haven't quite given up using them, but a support waypoint is only appropriate if the support function is a minor part of the mission. If it's important, then you need to apply some scripting to make things work well.
Remember that the unit doens't have to be on a support waypoint to help you. Moreover, they will sometimes move out in response to a call when waiting at an ordinary waypoint. I suspect if they are close to you they are not very good at responding. If you are further away its better, though if the injured unit is too far away they can forget where they are supposed to be going.
Tell us lots more about the exact circumstances and we'll try and help.