After reading all of your replies, there are a couple of more things to go through.
I assume that you have the correct belt. Since, you stated that it’s been hard to manipulate from early on.
Everything moves freely on the deck. Evidently, nothing is wrong there.
Now to the cable, cable bracket and lever. You need to determine if the cable moves freely in it’s sleeve. Disconnect the cable at the deck and move the lever back and forth. Does it move freely? Is the lever mechanism binding up? You may need to lube your cable or replace. You may need to replace the bracket assembly, where the handle is mounted.