Screen problem in Module pool program

Hi all,

I have developed a module pool program, in which i have used simple selection screen and after executing it will call screen 9001 to display the data. everything is goin fine but if i leave my output screen as it is for some time (or even if i lock my system then unlock it) i am getting an error message

"Selection screen ZDEV1110PGM_SF 1000 was not called using CALL SELECTION-SCREEN".

do any one have idea why this is happining without doing anything further to the output.

Regards,

Nilanjana