Any lumps of setup plaster in bags or storage bin should be removed. Clean all mixing equipment to remove set plaster or other contamination. Dirty equipment accelerates setting time. Check plaster by making a mix with distilled water. Plaster should always be mixed with water fit to drink.
Plaster unusually slow in setting.
If not contaminated, fault could be improper formulation for setting time by manufacturer. Lumps ranging in size from pellets to rocks are in plaster before mixing. Plaster has become wet en route to your plant. Bags showing evidence of water should be examined before carrier is relieved of responsibility. Carrier inspection should be initiated as the carrier is responsible for this damage.