Prints get bad in 2nd half of filament roll

Hi, Im printing batches of tank track links and when I just install new roll of filament (petg), the first one and a half batch (about 12 hours of printing) print ideal. Then at some point layer skipping starts and the prints become unusable (form is still there, but material is so weak because of missing layers that you can break the figure by hand easily). I noticed this always happens when there is less than half a roll of filament remaining. Any ideas what could be wrong?