If you've loaded filament you know is the correct diameter and type for your machine, and you're still hearing skipping or seeing weak parts there are a few causes
- Filament trying to extrude too fast
- Filament trying to extrude at too low of a temperature
- Burnt/jammed/bad PTFE liner
- Clogged/obstructed nozzle
Bad PTFE Liner:
You'll see here how there's severe discoloration in the PTFE (teflon) liner that goes inside of the nozzle. This could be because the printer was set to a temperature outside of its suggested or normal range (anything above 240* C for more than a few minutes will scorch the PTFE and damage the printer, and for extended periods of time - can emit harmful fumes). This could also just happen over time from printing alot. Flashforge printers include one (1) extra PTFE liner with them when they ship. The portion shipped is longer than what is needed, and has to be trimmed down to match the old portion that was taken out of the printer. This video starting at 1:33 shows the hotend being disassembled.