Adding a wax?

technically speaking, if the belt is not properly lubricated, it can "stick" to the deck causing that slipping feeling (technically it slips on the rollers). Usually the slipping on the rollers means the belt needs to be tightened.
As a rule the following belts (before the last year or 2) needed to be waxed somehow
Startrac
Lifefitness
Technogym
Nordic Trac

Preco, cybex and many others dont (usually if they do the display has a reminder every so many miles. Nationgym supply can tell you if you need wax and has it to sell if you do. The serial number helps here.

Tightening the belt is more likely the problem though. The rear roller usually has 2 bolts (usually its allen wrenches you need to tighten them with). To tighten, twist both clockwize a quarter turn at a time (do the exact same amount on both sides) with the belt running. Keep testing till it stops slipping. If this does not work then there is a chance that the drive belt (located under the hood that goes from the motor to the front roller) needs to be tightened. If there is an idler pulley then the belt wont be loose. If you do need to tighten it then somehow you have to move the motor away from front roller a small amount (worst case scenario you need to replace the belt). DONT tighten this belt too much as it can break the motor shaft. You can tell if its already tight by pushing on it and seeing if it deflects much (say a quarter inch or less is tight probably). Also running your finger on the inside surface of the belt tells a lot. If there is a lot of black dust (almost like soot) then the belt is pretty worn. To tell if the walking belt is worn jam your hand underneath and feel the surface (that hits the deck) if it is smooth (it should be closer to a waffle pattern etc) then it is worn, if the deck is worn through, you will need a flashlight to check, then that will cause the belt to stick. Another check is to walk on the belt only the outside edges and see if it moves smoother. the belt and deck wear in the middle.

I fix this stuff for a living. I hope this helps.

Bryan
July 2007