Tighten up the little Allen bolt in the middle. Put axle in,  tighten up. Ride bike,  take wheel out ,then  if I put the wheel back in the lever is totally floppy.

What am I doing wrong?

Have a bump.

Wild suggestions follow ;

Is the little Allen bolt vibrating loose?

Could you get a spot of thread lock in there?

Tighten up the little Allen bolt in the middle.

Have another bump ^^ what means this?

Put axle in, tighten up. Ride bike, take wheel out ,

You snapped the axle in order to take the wheel out without removing it?

what means this?

Maxle’s have a little (2.5-3mm) allen bolt in the middle of the can-side end. This can be tightened or loosened so that when the lever is flipped it stays closed (as not all bikes/forks have the same dropout widths). You can just see it in the middle where the pivot is (if the img works). On forks, the rebound dial can be removed as the shaft is the same size as the hex bolt.

https://media.alltricks.com/hd/rsmaxlelite.jpg

Maxle Ultimate do not have this