AnyCubic Delta Kossel Nozzle jam problem

Hi all a few days ago I tried to change the nozzle for the first time. after doing that I got issues like clogging in heatblock or nozzle, which with few times of disassembling parts and put them together fixed, but there is a problem still bothering. either of the old and new nozzles got jammed while printing. sometimes after a few hours or just a few minutes. I've checked the entire hotend for a clue of clogged but there was nothing. the only thing I've noticed is that the tip of the filament at the hotend becomes thicker! obviously, that's a reason why the nozzle jam but I don't understand what's makes that to happen.here's an image for tip of the jammed filament if it can help Thanks a lot

was there a little plastic ptfe tube in the hotend when you removed the original nozzle ?
If so it;s either not there any more or it;s been pushed up the heatblock.

The other thing it might be, is that the new nozzle ha sa shorter thread than the original - not all nozzles are the same.
In fact there seems to be at least 3 or 4 different designs. So just check that.
If the thread on the new nozzle is shorter than you need a longer guide tube.

Basically what's happening is th filament should be guided by a narrow tube to the top of the thread on the nozzle - and for some reason you now have a gap in the feed system before the molten plastic hits the nozzle.

That causes the lump at the end of the filament and really buggers things up - as you've discovered :-)