Snowblower throwing drive chain?

I have a 12hp 33" Yard and Garden snowblower. It is constantly
throwing the drive chain on the wheel axle. It appears to
be real loose. I see no way to adjust the tension.
I have loosened the drive axele and tried to adjust up but
it doesn't have slots to do that.
I have asked in another forum and the best guess so far
is a stretched chain. The unit is about 5 years old I'm guessing,
I bought it used this fall.
any other ideas? Thanks.