Sounds kind of like my '74ish Ariens. The "front" lever engages the impeller/ auger drive. I have found that the coupling under the plastic shield sometimes gets either frozen or basically tries to engage, but gets to a "male" to "male" coupling situation -you should remove the shield to see what's happening - ENGINE OFF at first.
My Ariens also has two different belts, one for the auger/ impeller and one for the wheels. If you can engage the wheels (independent of the auger/ impeller) then perhaps only the auger belt is gone (or loose?). However, The fact that you stated that this occurred a few times last year but rectified itself leads me to think that it is a coupling engagement problem. Perhaps the engagement thing needs lube.
See if the engagement thing is moving/ engaging. You should also be able to figure out at this point if the belt is toast or not.
If nothing is obvious, start the engine (without the cover) and CAREFULLY observe what is or is not happening.
Good Luck.
Ralph