When the adhesive is incapable of being troweled satisfactorily; the adhesive "balls up" under the trowel. Balling up can occur when foreign material is mixed into the adhesive while troweling causing the adhesive to thicken. Possible causes include:
Dusty, dirty substrate.
Picking up patch when the entire subfloor is skim-coated with a patch material.
Weak, friable concrete which breaks apart when troweling.
Unstable adhesive - possibly frozen.
If due to foreign matter getting troweled into the adhesive, the subfloor must be cleaned of all dust, dirt, and debris. Skim coating the entire floor, while a common practice in some areas of the country, is not necessary. Only those areas requiring repair should be patched. If balling up is due to the adhesive being frozen, and the adhesive has been stirred down, the adhesive is no longer usable.