Prints not flat

My prints never seem to come out with a flat bottom, they always seem to curl upward a little.

The curvature is very minor and it's really not noticeable by eye but whenever I attempt to join two prints together the (supposedly) flat surfaces are both slightly curved meaning the joins always end up with a slight gap around the edges. Any idea how to fix this?

Note the prints always stick to the bed well, and no amount of skirts or bed adhesion or anything seems to fix the issue.

I've tried a whole range of temperatures and settings and while I generally print with eSun's PLA+ the problem seems to be notably worst with PETG but even PLA has the issue.

At first I thought my bed must not be straight but after checking with a straight edge it seems the bed is flat, it's just my prints that are not even though they seem to stick to the bed perfectly.

At the moment I'm trying to print a puzzle box and the pieces just won't fit together properly. It's most annoying! Any idea what's going on and how I can fix it?


I'm using an Ender3 with PLA+ filament.