If is it is slipping or grinding: Check that your nozzle is clean (a common problem is that the bowden quick coupling breaks and small piece of metal is pulled into the nozzle. It obstructs plastic flow there. The most easy way to check this is to remove the bowden and let printer heat up the nozzle. Then try to push the filament into the nozzle by hand. It must go really easily. (The plastics must flow quickly with a small force of your hand.) If it does not go easily then you need to clean up the nozzle.
Now you need to find out whether it is slipping or skipping. Stop the print when the slip/skip happens and retract it. If the filament is ground then it is slipping.
If it is slipping: Increase the force pressing the filament against pulley. If it does not help or if it squashes filament a lot then you need to replace pulley for something better. Too squashed filament will increase friction in the bowden and the result is .... slipping or skipping.
If it is skipping: Increase the current on the stepper driver for the extruder motor to the maximum the driver/motor can handle. If it does not help then get a geared extruder (best gear ratio is in the rage of 1:2 - 1:3).
