Insuffient agitation and spin

I recently put new brake shoes on the washer and replaced the belt and idler pulley. The washer doesnt agitate strong enough and the spin cycle doesnt go fast enough. I can see the belt doesnt look tight enough and the drive pulley spins but the belt slips. Is there a way to adjust the belt tighter or could there be another issue? Amana model: LWD70AW

I went to the diagram of how the belt should be routed. I had it incorrectly routed. all I did was put the belt on the other side of the idler pulley and the belt got tighter and now the agitation and spin work awesome.