It might not be the signal that is the problem. It may be that what you are asking the person to do is a foreign concept to them, due to their inexperience. Let's use the signal for "cut that branch" as an example, ok?
Say you are on the ground, directing the job, and want the climber in the tree to cut a particular limb. If the bucket truck engine is running, or an air conditioner is on, they can't hear you very well. So you signal.
If a limb forks close to the trunk, and the lower section is too close to the ground, another tree, or whatever, it can be better to cut half of it off, rather that thin it. The climber is obliviously thining away, which is a complete waste of time, since that limb is going to have to come off, as soon as you can get their attention.
Trying to communicate this with hand signals has little to do with the particular sign language you select, but with the fact that the climber has missed the point of this particular job. Somehow, in the walkthrough, or the pre-climb inspection, they did not notice the opportunity for a big timesaver by making a big cut early in the pruning operation. These big cuts need to be made right off the bat, because they clear large areas for inspection, access, and fine pruning, if necessary.
I guess the point of all this is that, in many cases, it is not the unfamiliarity with the signal, it is the unfamiliarity with their job that is the problem.