This is a fascinating problem. Usually, button clicking is one of the most problem free functions of TC. Full disclosure, I haven't done keyword testing in a long time.
In the keyword test I would try using "Click", "ClickButton" and "DoDefaultAction". They are similar button different.
Also, is there a slider control you can use instead to accomplish the zooming? Some applications have both. If there is a slider maybe you can accomplish the same thing with it. Some applications also have keyboard shortcuts for zooming (e.g. <Ctrl> +). I don't know if that applies to yours. If it does, that might be another option.
