Interact With Target, is a Blizzard function.
For best effect, also use Click to Move (Interface - Mouse Options).

I personally have one mouse button as assist (using Target=TargetTarget, for my DPS macro's).
The other mouse button is my Interact With Target.

I'm not sure why it would only work half of the time for you.

Verify each step along the way:
1) You're targeting the main.
2) You assist the main so you have their target.
3) You click the interact keybind.
4) Your toons move towards the target.
5) They interact when they get there.

Not really sure what else I can say.
If you're using Jamba and follow strobe is enabled, there's a chance you interact with a target, but then Jamba strobes follow which breaks that. Not sure if this is the case or not.