The insufficient temperature in the barrel and die
A higher level of filler and stabilizer. Also a higher amount of external lubricants such as wax.
Gelation promoters are absent in the formulation
Check the functioning of the temperature controller and check the stock temperature of the extrudate coming out from the die (melt temperature)
Reduce the level of filler and stabilizer. Keep the optimum level of external lubricants.
Use process aid and calcium stearate or secondary plasticizers depending upon the application of the products.