Good day all, newbie here. We bought a used Ender 3 v2 a year ago or so. I think the previous owner changed the hotend. It takes a slightly different start gcode. Anywho, as of late we have a lot of trouble with prints, especially starting out. Running polymaker pla, 195 temp. We’ve been setting the Z-offset at .05 or so, as it seems the nozzle has drug at times. After running the purge, it will keep extruding while it moves to the center of the plate to start the print. The extruder gear doesn’t turn, but it keeps drawing a line. Then when the print starts it takes the extruder a bit to catch up, so the first layer is incomplete. Occasionally it will string/blob which messes up the following layers. We just replaced the nozzle a bit ago. I don’t know much about printers so you might have to type slowly! 
You’ve got to determine if your printer is under extruding or over extruding. You have also got to set up your printer’s retraction, as well as your Z axis offset.
Thanks. We’re using Cura for a slicer. Retraction is enabled, but I don’t have z hop enabled. Could you explain over/under extruding a bit more? And is there a calibration or something for that?
Check these 2 videos. You’ll get the point.
Thanks for the videos. Our Z axis offset is/was set at 0. We had just leveled the bed not long ago. A piece of paper fits about right. Our printer doesn’t let us go negative Z axis offset.
On the second video, our printer is an Ender 3 V2, and the software is different. I set the extruder to 100, and it only extruded about 20 mm. The transmission ratio for E was 93. I set as far as it would go to 186 but it didn’t change anything.
Thanks for taking the time to help us!