About LOOP AT SCREEN

I want to add some code to user exit USEREXIT_FIELD_MODIFICATION in TCODE VA01 for setting screen attribute of field VBAP-KDMAT.

But in the PBO module MODULE FELDAUSWAHL OUTPUT, the field VBAP-KDMAT won't execute this user exit. But other field in the same screen with VBAP-KDMAT can execute this user exit. So once step into this user exit ,i loop at current screen(NO:4900).And change the attribute of field VBAP-KDMAT.But after execute this user exit. The loop at screen in module MODULE FELDAUSWAHL OUTPUT lose its correct index.The next screen element will not change any more.

This is a nested loop at screen. I guess whether nested loop at screen will work.

Can some explain it to me? Thanks.