That happened to me, I've changed the teflon tube for a tighter one that I had laying arround and helped. But the best improvement was to decrease the speed between 60 and 80 mm/s.I've also thought that increasing the temperature will increase the extrusion speed, but after many tries It never worked; it just clogged the nozzle.Also check if your extruder makes a clicking noise, that's a proof that your printer is going too fast.