You need to clean the heater throat (first take extruder apart and of and take the nozzle of) that allows the filament through by heating up a round iron wire and push that through so attached restfilament is pushed out. be careful when doing that. after that the filament will glide through much easier.
You have to use a bigger round iron wire that fits the heaterthroat diameter
I had that same problem last month and it worked for me.
there is a video somewhere to be found as guide