Make sure the filament is wound properly on its spool - specifically that it is not wound with overlaps that can stop the extruder gear from feeding filament. I had this problem with one spool and had to completely re-wind it onto another one.

Also make sure the "teeth" in your extruder gear are not clogged. There should be zero slippage when the gear feeds filament. Sometimes the gear teethgets partially filled up with filament dust and this can cause miss-feeds - or no feeds.