I had this problem initially (stiill do with abs which is why I'm printing in pla atm). For PLA i found some advice which recommended compressing the extruder springs so tight that the extruder motor stalls and then backing off, and this has worked for me (one thing to watch out for is having your nozzle too close to your bed which makes it almost impossible to extrude anything and causes the filament to strip. ABS as far as I understand is much softer so you need a lot less pressure so you can't take the approach of stall the back off, so you just have to find a happy spring compression that doesn't strip (by trial and error cleaning your hobbed bolt each time
) If I'm wrong about ABS I'd love to hear though I'm waiting for my next attempt till I have a second hotend.