And other rare things to check
1) engine bolted down tight to mower
2) belt guard in proper place around engine spindle.
And of course correct belt routing.
If a pulley is flat the back of the belt runs on it , if it is V then the inside of the belt runs on it.
Easy to run the clutch pulleys backwards.
Once a belt has run off a few times they get distorted and will jump off with the slightest provocation.
If the pulleys are worn deeply you might need to go down a 1/2" or more in belt size.
The correct size belt assumes you have correct ( new ) size pulleys and correct ( new & strong ) springs.
It also assumes all the pivot holes are nice & round.
all of the above can conspire to make your mowing hell.