This might have already been answered - but - My wife asked me to look at her no no model 8800.
First I removed the replacement tip and could see that the little rubber drive belt was stretched to the point that it was not working.
I used a very small flat tip screw driver and removed the outer colored cover starting from the tip end(came off with little effort). Next I removed the inner gray colored case the same way.
Then I used my fingers to pull off the little white colored wide roller that makes contact with your skin. Next I popped out the tiny motor with the same screw driver which has the drive pulley and holds the bottom of the drive belt (little rubber band).
For a replacement belt I was at first going to use a small o-ring - but- when I compared the elasticity of the o-ring to the original belt, the o-ring did not have enough stretch.
I told my wife the situation and she mentioned that our daughter still had some elastic dental bands hanging around and that they were very "stretchy".
So i used - and I quote - ( ORTHODONTIC ELASTICS 1/4" HEAVY NON- LATEX ).
At first sight these will appear too small but they stretch perfectly ( and your original will be already stretched out much longer than it was from contact with oils from your skin.
Put the new band around the pulley wheel of the little motor ( before ) you snap it back in place.
Now stretch the other end of the band over the pulley end of the wide white roller bar just before you snap this back into place. Now replace the inner and outer covers and the replacement tip and you are ready to go.
The dental elastics are square edged instead of round like the original bad - but it will not affect the performance.
I hope this was helpful.