Undercuts in mold and possible surface imperfections. If undercut is intentional, redesign or reduce. Redesign runners and gates for proper filling of cavities. Mold parts too hot for ejection. Increase cooling time.
Decrease melt temperature.
Decrease mold temperature. Molded parts sticking to stationary half of the mold. Redesign sprue puller. Apply mold release. Increase nozzle temperature.