1. There is impurity in the filament which got stuck in the nozzle. Please clean  the nozzle using a needle or take the nozzle off and clean inside of the nozzle  using needle and small drill. 


2. The nozzle is overheated causing the filament to carbonize inside.  Please clean the nozzle by needle; or take the nozzle off and clean inside of the  nozzle by needle and drill. 


3. The deformation of the nozzle hole occurred by external force.  Please replace the nozzle.