Troy Bilt Mustang xp 42 2017

Check all pulleys and spindles to make sure all are good and no roughness/loose, worn etc.
 
There's also a bracket, pulled by a heavy spring to help keep tension on the belt (see below link/pic's).
If that rusts up (not unusual) and doesn't move that can allow the belt to not become fully tightened. 
 
It's best to pull the deck and inspect/lube that part but a squirt on that part may help. 
Obviously, don't spray on the pulleys, etc..
 
 
https://jackssmallengines...ustang-xp-42-2017/deck