The tensioner will only put tension on one side of the belt. The tension on the other side is provided by the teeth on the cam gear and crank gear.
When tensioning you need to line it up, fit the belt, then turn the crank ever so slightly whilst keeping the camshaft still - this will tighten up the front side of the belt, then you nip up the tensioner to pull in the tension on the back of the belt.
Not 100% sure whether you have the room to get to the tensioner with the bottom cover on, IIRC I didn't think you could.