You write long macros and you use features that I've never known existed. Good work.
You aren't using double click. You aren't using click and drag. You aren't using Move mouse. These are the things that would cause timing problems. You aren't using them.
My guess is that the small delay between finding the image and clicking on it is where the problem lies. Your images are quite small and even if KM is taking only 0.01 seconds to find the image, there could be a slight miss on the click if the Wacom tablet is moving. The KM ability to find images is extremely fast but not instant. One way to speed up the search is not to search an entire window but just a small part of the window. I don't know if you can make that change in your code because I don't know if those images are fixed to a certain location or if it can be anywhere in the screen. You could test out this idea by seeing if the interference only exists when you move the Wacom pen faster at a certain minimum speed.
Another way to test is to keep the Wacom pen still but try moving the mouse a lot. At a certain threshold I think even the mouse would mess you up.
Any further thoughts? I'm not sure I can do more to help.