we do this all of the time and I am familiar with your problem. Depending upon what you are doing, you will need to do one or more of the following:
a) change your dspf file to CHGDSPF xxxxx RSTDSP(*YES)
b) close and open the display file (USROPN option) when you start the program displaying the window (assuming you are not using INLR in this program; we do not use INLR on our window programs). Failure to do this might display a background to the window, but it will be an old version of the background.