You may have something bussied out on the standby side.
If you "status PNC" you should show warnings if there's smething bussied out. Confirm that at the top of the page "Standby Busied" says "no"
Do a display errors and specify error code 18(busy condition).
If you have any PNC components (i.e. EI boards, SNI boards, or DS1C's) on the standby side that are bussied out, you'll have to release them first.
If you do have something busied-out, it would be worth investigating why it was busied-out before interchanging to that side.