Turn the shoulder bracket to about 90 degrees then push up or down. It'll take a bit of muscles but it can be done. When you have it where you want it, be sure to pull down on the belt as hard as you can to ensure it is in the channel. There's like 4-6 levels in will lock into.
If you are still having trouble, take the cover off so you can see how it works. That'll help a lot.