I posted this previously for others, but here it is again.
It might be useful to know which hot end design you are using.
You need to determine what the real culprit is.
1. Using too high a temperature, and/or thermal barrier hole too narrow.
PLA in particular expands quite a bit when heated near the melting point. It also becomes very soft long before it melts. If the thermal barrier tube diameter is too narrow, the filament can seize against the walls as it warms up.
To test for this condition, remove the hobbed bolt, and try pushing filament through by hand. If the temperature is OK, you should be able to push through with ease. If you cannot easily push the filament through, try pulling it back out. If it is stuck, then your filament has seized in the termal barrier. If you are brave enough, try drilling the barrier out in diameter increments of 0.5mm. You should not need to exceed 4.0mm diameter.
DO NOT proceed to adjust roller bearing pressure against the hobbed bolt until you can push filament through by hand. You will never solve the problem.
2. Using too low a temparature.
I don't think this is the case. Easy enough to check, right?
3. Bearing against the hobbed bolt too loose.
If you were able to push filament through by hand with ease, and if the hobbing is full of plastic dust, then your roller bearing is not tight enough against the filament. Tighten it beyond the point where the big gear can move, and slowly back off the pressure until the filament begins extruding.
4. Bearing against the hobbed bolt too tight.
If the bearing is too tight, the stepper motor should stall, and lash the big extruder gear backwards at regular intervals when you try to extrude. Slowly back off the pressure until the filament begins extruding.
