Sometimes the spring pressure isn't enough to fully tighten the belt so you have to finish it off manually. Loosen the 13mm headed nut on the tensioner and loosen the splined spring holder bolt as well then push the pulley out manually until the belt is tighter. If you can't push it enought use a lever or a long sturdy screwdriver to push on the tensioner base plate until the belt is tightened then quickly tighten up the bolts before it slips back.
The belt is tensioned about right if you can just twist it quarter of a turn hafway between the cam and jackshaft sprockets. Once you are happy it's tensioned OK turn the engine over a couple of turns using a 19mm spanner on the bottom pulley then check the timing marks still all line up.