This seems like a problem that I have had with the extruder, check the opposite end of the bowden tube - make sure that the connector is not broken (it can happen to the OEM extruder that comes with the printer). If this is the case I'd suggest replacing the extruder with an all metal one, amazon sells them for about $20 and up.

The nozzles can be replaced with any that you find on amazon that fit the heat block - Try these