You'd be amazed how much gunk can fall into the hopper when the manufacture extrudes the filament, usually dust and airborne particles. On windy days even tiny wood chips and leaves etc. Something like that gets into your nozzle it requires very intense cleaning to remove unfortunately.

- If using ABS only: soak for an extended period in acetone. Then clean out with a toothpick. Repeat multiple times.
- If using HIPS only: soak for an extended period in D-Limonene. Then clean out with a toothpick. Repeat multiple times.
- If using PLA only: this is a tough one. Heat is the only thing that works here but PLA will char and burn to a hard lump when overheated exacerbating the problem.

If you have the 'feel' for it you can buy a 0.4mm drill bit and drill out the nozzle by hand. Be aware though that carelessness will ruin your nozzle.

Good luck.