Top loss of traction cause we have seen....1-overlubracation of hoist ropes. (clean with mineral spirits)...2-car out of balance...3 undersized cables (even when cables pass GO/NoGo)We see a lot of this especially on Northern Machines. Even basement set with 180 degrees of wrap.... 4 Sheave wear (mostly 300fpm or higher Westinghouse machines/some Otis and Haughton. You must replace sheave or machine)....5 Otis with sheave liners ( must be lang lay ropes and must have generous amount of Otis vitalube cable lubricant applied on new ropes.And ONLY this lubricant!!(Yes adding oil to ropes stops loss of traction??? Gave up trying to figure this one out....Good Luck out there!