Installed new bottom threshold, door won't stay closed

I have a 1/2 hp craftsman belt driven opener.  I added a new bottom threshold (about 1 inch thick), and the door will not stay closed.  It goes all the way down, then comes right back up.  I tried adjusting the force and down stop, nothing worked.  I've adjusted a chain driven opener by moving the pin a link back, but can't find anything on how to adjust a belt driven opener aside from turning the adjustment knobs. 

Any thoughts?

You need to adjust the down travel limit not the down force. Every opener is a little different so you should refer to your operator manual on how to do this. If you tried adjusting the down limit and it did not respond then there's possibly an issue with the down limits.

already tried that, didn't work. Any way to adjust the belt itself?