Help: EXTEST (boundary scan) for EPM240

I encounter an issue when executing EXTEST (boundary scan test) for EPM240. If there is no code(.pof) in EPM240, I can use EXTEST instruction to let IO output high or low level. But if it has code, I cannot control IO status. Moreover, if unused pins are configurated as output driving ground, can we control these unused pins output high or low level by use of EXTEST? If anyone meets this similar isse or know what's problem happened? Could you share your undstanding? Thanks.