This may work for you...at another time, with a similar setup, it worked for me. Give everything on and around the tensioner (except the face in contact with the belt) a generous dose of penetrating oil ...WD40 could do, in a pinch. They are prone to locking themselves into one position, unless lubricated from time to time. My method used to be...douse the parts which should move, with the light oil, wait an hour, give it a couple of good taps with a rubber mallet and repeat the oil application.
Give it a try.