I had a similar occurrence of of slippage with my #30 jacobs handpiece...
I waas going to order a new one (pink!) but decided to call the wonderful people at Rio Grande Tech support first...
It was a good thing I did!
The support technician walked me through taking apart my #30 handpiece and putting it back together.
It seems that a part got dislocated in there and was not seated properly.
We got it all seated nicely and put it back together and it worked perfectly after that.
The hand piece twists apart...if you look closely, there is a faint line of separation between the two sides...mine was verrry tight...I had to use a wrench and vise to assist in unscrewing the two sides.
(when I looked down into the handpiece before I took it apart, I could see a washer-like disc sort of moving around...)
anyway...@riogrande got me all squared away!
here is a foredom handpiece maintenance video:
here is a foredom flexible shaft maintenance video:
here is a video on foredom motor maintanence and motor brush replacement: