I have had a Ender-3 3d printer for a while now, and in the past could have months it has been printing horribly. Some times it starts fine, but then leaves gaps and weak layers that look stringy and incomplete. I have researched this and I am pretty sure it is under extrusion. I have tried many things to fix this such as more nozzle heat, increased the flow, and decreased the printing speed, but nothing has worked. I also noticed that when the filament comes out it is pockmarked and not smooth. I have had the PLA that I am using for about 2 years, is it possible that the filament can get old and needs to be replaced. I have attached a picture of what the problem looks like and any suggestions on how to fix it would be appreciated.