Troy Bilt Bronco Automatic transmission - variable speed pulley falls off

Hi everyone, looking for any advice and trying to see if someone else is having this same problem. I have had my Troy Bilt Automatic Bronco for 3 years and the variable speed drive pulley has fallen off 4 times. Is this a common problem with this model or just for me? How can I repair this? Thanks for any advice or experience you can share.