You definitely need to do an annual check of the drive box innards - this isn't very difficult - with a mostly empty gas tank, just stand the machine up vertically on the auger housing (yes it will balance there, and no this won't hurt the engine... (If there is a battery in your unit, remove it as well) You can then remove the plate from the bottom of the drive box and look at the innards.
You will see a fair number of shafts with gears and chains, etc. Most of these should have some grease or lubrication with one big exception. This is the drive plate and disk. The way the tranny works is the engine drives a large flat plate which spins at a fixed speed determined by the engine RPM and the ratio of the various pulleys between the engine and the drive plate. Sitting at right angles to the drive plate is the drive disk; this is mounted on a splined shaft that allows it to slide back and forth with respect to the drive plate. The disk has a sort of rubber tire around it's rim, that the clutch brings into contact with the drive plate. The disk connects to the drive wheels via the other drive train parts. Because the angular velocity of the drive plate changes (and reverses direction as you cross it's center) the transmission gear is determined by where on the splined shaft the drive disk is positioned. (This will make more sense when you look at it...)
There are essentially 2-3 major failure issues that this type of transmission experiences - first is the general issue of needing to be lubed, however one should be careful to NOT get lubricant on the drive plate or the tire on the driven disk - this can cause slippage.
Second is the clutch needing adjustment to fully mash the drive disk all the way into firm contact with the drive plate.
Third is that the drive disk is a wear item - that tire wears over time and eventually needs replacement - frequency varies considerably with the use / abuse the machine gets, frequency and amount of snow, etc... It is not a terribly difficult repair, and the part is not very expensive. The disks can go out with little warning, so IMHO it is worth keeping a spare in your parts kit, along with a set of belts and a bunch of drive pins.
Belts can also be an issue - loose or worn belts can cause either weak drive, or anemic snow throwing depending on the problem belt - again check for proper tension.
One other big improvement that I can't reccomend enough on any two stage snowblower is a
"Clarence Kit" - it is one of the best performance boosters I have ever encountered. Pre-Clarence, I did a good job clearing regular snow, but nothing spectacular, and slush or heavy stuff was problematic, especially if I was doing "re-blow" to get stuff that didn't get all the way out of the driveway the first time I moved it. Slush would only go a few feet and I got lots of clogs... Post Clarence, I was getting at least 4-5' greater height and distance with light snow (I was hitting trees that I had never even scared before :coolgrin: ) but the big benefit was with slush, which I was now able to throw about like dry snow would throw pre-kit, and almost totally eliminated clogging.... The kit is basically a set of rubber blades that bolt to the tips of your second stage impeller (the fan) and fill the gap between the blades and the blower housing. This increases the effective tip velocity of the blower, and leaves anything that the screw brings into the impeller no place to go but out the chute...
Hope this helps...
Gooserider