Windows handles screen repainting as a low priority task. Paint requests always are sent to the end of the queue and even then the request to draw/paint the screen is only done when the application itself asks for the next message from the event/message queue.
Generally the Windows version of PVXPlus will refresh the screen whenever it goes to user input, a Wait is issued, or 1000 instructions (whichever comes first).
If you are calling external DLLs they may not have a message/event processing loop to service Paint requests thus your screen may not get repainted.
As a general rule of thumb we suggest that you issue a dummy WAIT 0 prior to calling a DLL which will at least help by forcing the system to process all paint requests its screen prior the DLL call.