EDIT: This is for the ADV4

I posted this as a comment somewhere else but thought I would give it its own post.

Yes, this is a common problem.

Step 1: Calibrating extruder

https://www.thingiverse.com/thing:4838076/files

Set the following

Flashforge PLA

Layer height .20mm

First layer height 0.30mm

Shell count 1

Overlap perimeter 0%

Top solid layers 0

Bottom solid layers 3

Fill density 0%

Raft No

Brim fully enable

Extrusion ratio 100%

Cut off the top and bottom layers

With digital calipers measure each side diagonally both ways using the middle part of the caliper jaws not the tips

Add them all and divide by 8 to get the average

0.40 / result = percentage

Set extrusion percentage to what you calculcated and print again

If it measures correct proceed, if not adjust

Step 2: Bed Leveling

Use the auto calibration, and whatever method you want to determine height because we are not going to use these values we just need a reference -- whatever it is it must be consistent

Get all nine points and write them down: for instance

-3.30    -3.25    -3.05
-3.25    -3.15    -3.00
-3.45    -3.15    -3.00

Since these are negatives that means that -3.00 is the highest corner (rightmost towards door and rightmost middle). This is what you need to get everything to be level with.

You will need a shim -- it must be consistent and thin. I used 3M foil tape which has a thickness of 0.05mm with the foil and adhesive. Remember that raising any point will also raise adjacent points. I added 1 layer of foil tape under the entire left side and an extra layer square around the leftmost door-side screw hole, and one layer square on the rightmost back screw.

To shim you need to remove the bed and put the shim between the rail mounts and the bed, not above the plastic.

This brought me to:

-3.45    -3.45    -3.50
-3.40    -3.45    -3.45
-3.50    -3.45    -3.45

Yes I know it appears that everything went lower -- I have no idea what is going on but as long as you measure the height between the plate and the nozzle consistently on all points it doesn't matter what the numbers are, just their relation to each other.

Now the fun part begins.

https://www.printables.com/model/125939-9-point-bed-test/files

Use expert calibration mode -- we are done with the 9 point, it is useless

Set a reasonable number and try a test using the extrusion percent you got before

Keep print settings consistent between prints

Use an adhesive on the bed, you don't want bad print adhesion to mess you up

When you get all the squares to be relatively OK and you aren't thumping any more, you are done. Write down the number and tattoo it on your forehead so you don't have to do this again.