Take off the sliding jaw and clean the bar thoroughly. Be sure to get all the old glue and whatnot out of the little grooves in the bar. Use some solvent and a brass wire brush. (Steel might spark and ignite solvent fumes.) Also look inside the sliding jaw and try to clean out those grooves inside as well.
Finally, if you look inside the sliding jaw there are two set screws, one on each side. Try turning them 1/4 turn either way to see if that improves the grip. You can reach them with a long 3 mm allen wrench.
If all that doesn't work, contact Bessey and explain your problem. The K-body clamps have a limited lifetime warranty.