AnyCubic Delta Kossel Nozzle jam problem

[HomaUn]

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