Well I've just made an absolutely gut-wrenching discovery.
I've been fighting with adhesion issues on my 3D printer for months, and after making Z-Offset adjustments, temperature changes, leveling and re-leveling the bed countless times... I decided to check the extruder nozzle height at more than just the corners, and I've discovered the bed is warped. Fuck.
This is a manually-leveled machine so there's no way to compensate for this. I don't know what to do.
#3dprinting

@FyxTheProto On some of those older printers it was common to have to find (or make) a sheet of glass to match the heat bed size and clip it to the bed for a surface that is flatter than the metal bed. That or glass with a PEI sheet on top. That's what I ended up doing with my old Monoprice Select Mini v2 and my CR-10S years ago.

That's usually fairly cheap to do, especially if you have a hardware store nearby that can cut you a custom piece of glass.