One of the drawbacks of the MX6 is all of the moving parts wear out over time. Fortunately the parts are inexpensive and easy to install. I would say after 1 ½ to 2 years you will need to start to replace some parts, and this depends on the pool run time also. But figure at the 2 year mark replacing all of these parts featured in this video series would be a good idea.

The same part is used for both cleaners.
The first step thing to check if the MX6 is not moving are the Tracks. The Tracks are made out of rubber and have raised “teeth” on the inside that grip the plastic “teeth” on the wheels and this is what gives the cleaner it's awesome grip on the pool surface. When these runner “teeth” start to wear down the tracks will start to slip in the wheels and the MX6 will begin to move very slowly and may stop moving entirely. Here is the part you will need to order - 2PK Zodiac Baracuda MX8 MX6 Swimming pool Cleaner Wheel Track Tire R0526100

The engine is the heart of the MX6 and this is what spins to pick up the debris in your pool. Depending on your pool's run time you figure this thing is spinning everyday for 6 to 8 hours so after a couple of years it will start to slow down. The MX6 will still work fine, you will just notice that it moves slower than when you first dropped it in the pool. So replacing the Engine along with the Track is a good idea.
If you change the engine and the MX6 is still not moving at full speed maybe the Cyclonic Turbine is also worn out. They are all tied together, the Engine the Tracks and the Cyclonic Turbine so it would not be a bad idea to change all three parts at the same time (change both tracks at the same time).
Here are the other videos in the series: