Look Keo pedal no longer floats. Can it be fixed?
Put some soap on the cleat contact points to lubricate it.
Be careful not to get it everywhere, just in case it becomes too floaty!
This assumes you have covered a bunch of other options (of course!)

The cleats are actually the same and you didn't mix-n-match (some other manufacturers make compatible cleats which might be slightly different)
The black protruding insert is not different / damaged / inserted incorrectly.
All the caveats above regarding dirt etc.