I think that the driver is incapable of generating a touch message in this case because touching with a single finger can be the start of several scenarios.
The driver is obliged to wait for you to either :
- lift the finger for it to generate a right-mouse-click, or
- move the finger for it to generate left-mouse-down and left-mouse-move, or
- touch with another finger for it to generate one of several multi-finger messages.
You could try to update your touch device driver to the latest version, but I think that the driver has no choice in this case but to wait for a follow-up action before generating any sort of message.
My advice is to change your planned user interface so as not to use press-and-hold as a gesture.
I haven't been able to find any documentation to support my above ideas, so it is to be taken only as a conjuncture about the functioning of Windows touch drivers.