Before you go getting your itomaki all sticky, confirm that you are tuning properly. You may be doing this already, but you need to use your thumb as an anchor and get a tight fit. The easiest way is to over tune, then turn it back the other way, and really use your thumb anchor&d in the tenjin to get a tight fit.
If you are already doing it properly and its slipping, then yeah the best thing is to get a shamisen store to re lathe them, or use the violin resin method as a stop gap.