This is a bug in 9.1.
I have managed to get it to work by displaying the UDF area, then moving the UDF area to a different location (side or bottom), then closing form. Re open form, close UDF area & maximise. Once you have it right, log off SAP using File>Exit to make sure settings are saved, then log back on to check.
Alternatively, upgrade to patch 8, I think it is fixed in that, but do check the SAP notes to make sure