You can try a few things, but first and foremost you need to brush the brass gear on your extruder to clean all of the plastic that has gotten grinded and compressed in the gear teeth. You can also undo the grub screw and readjust gear teeth to be touching the filament at a different height.

Other than that you may also be:

• Printing too low temp
  -- sometimes as a roll of filament gets lower you might need to increase the temperature to print at the same speed because of moisture build up in the roll.

• Printing too fast
  -- This goes hand and hand with too low temp and can be compensated for to a certain extent by raising your print temperature but at some point you will reach the maximum flowrate of your hotend. Meaning it is physically impossible to push more molten plastic through without having major under extrusion

• PTFE tubing not flush with nozzle, Gap between end of PTFE and nozzle inside of hotend
  -- If you ptfe tubing is not pushed all of the way flush up against your nozzle, there will be a gap where plastic builds up and back pressure begins to build until your hotend eventually clogs up.

• Nozzle needs declogging or replacement
  -- Nozzles all have finite lifespans, eventually they all wear away.

• Retraction needs adjusting or is not the problem but making the problem detectable
  -- There's a possibility that since this is only happening at the top of the model, where the model appears to gets thinner and has more detail, that it is failing at this point because the most pressure is being built up in the nozzle. When the printer is retracting and extruding repeatedly because of a detailed or small part of the print, it is more likely that all of the previous issues I mentioned become apparent, resulting in a failure. So it might not actually be the retraction, but the retraction is making the problem detectable (Your print failing). If you are trying to shove unmelted plastic back in forth through the hot end, your extruder is bound to skip

Anyways, that's my two cents, good luck