I have run into this problem before on my 600SX trail Mod.[now a 144"] We trail rode and drag raced this sled in improve stock from 1999-2007, it has over 9000km on it now. With my sled it had enough drag races on it that the belt had worn a ridge just around the engagement area of the primary clutch. On engagement it would grab the belt so hard that it would pull the belt down in the secondary, bog and slip, it became impossible to launch...had to replace the primary clutch to fix it. My 700SX trail mod [144"]was doing the same thing the other day when I was running my SRX against it,[I use the 700sx as a benchmark to set up the other sleds], I guess its time to change the primary on the 700SX as well. High clutch engagements are bad for wearing a groove...one of the downfalls of the hard compound near bullit proof Yamaha belts...at least thats my experiences.
If you have a groove warn in your primary sheaves , swap the clutch with someone elses and see if it cures it.