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?

It sounds like the addition of the new bottom threshold may be causing an issue with the door's operation. Here are a few thoughts and suggestions to troubleshoot the problem with your Craftsman belt-driven opener:

Check the sensors: Make sure the safety sensors on either side of the garage door are properly aligned and not obstructed. If there is any misalignment or obstruction, it can prevent the door from closing properly. Clean the sensors and ensure they are facing each other without any obstructions.
Check the force settings: Although you mentioned you adjusted the force, it's worth revisiting this step. The force controls how much pressure the opener uses to close the door. If the force is set too low, the opener may detect an obstruction (in this case, the new threshold) and reverse the door. Try increasing the force gradually and test if the door stays closed.
Check the travel limits: The travel limits control the distance the door travels when opening or closing. If the travel limits are not properly set, it can cause the door to reverse prematurely. Adjust the down limit by turning the adjustment knob in small increments, following the manufacturer's instructions. Test the door after each adjustment to see if it stays closed.
Consider adjusting the force sensitivity: Some belt-driven openers have settings for force sensitivity. These settings determine how sensitive the opener is to detecting obstructions. If your opener has this feature, consult the manufacturer's manual to see if you can adjust the force sensitivity to accommodate the new threshold.
Consult the manufacturer or a professional: If you have tried the above steps and are still experiencing issues, it may be helpful to contact the manufacturer for specific guidance or consider consulting a professional garage door technician who can diagnose and resolve the problem.

If possible, we are happy to provide online guidance and help, you can contact us:
CHI Hardware Co., Ltd.

Or you can directly send us an email about your problem, and we will reply to you as soon as possible after seeing it.
Email: chi@chihw.com