Snow Blowers

I have a 9-26/E. Fifteen years old(12/97). When the snow is wet or slushy the drive wheels do not turn. Damp or powder snow and it works fine. What is the way to fix this condition. It had been happening for many years but seems to be worst now that I am older and can't push it like I used to. Please help.
Thank You in advance,
Guy Beauvais