SPD pedal float becomes restricted I have a strange problem with my clipless pedal float becoming restricted. I recently installed Shimano M520 SPD pedals on my bike, with SM-SH51 cleats on SixSixOne Expert MTB shoes. A few weeks after initial set-up, the right side started not floating correctly. The float range felt reduced, and even within that range the float felt like it had more friction/restriction and didn't glide like the other side. A very experienced (40+ years) bike fitter looked at it and was completely perplexed. We switched out the pedal -- no difference. Switched out the cleat and the problem seemed to be fixed. The only noticeable cleat wear was in the center of the trailing tab thing, on the surface facing down when pedaling (strangely, from what I can see, that part doesn't contact the pedal when clipped in).

It's now been 1 1/2 weeks since the cleat was installed, and the problem has returned (got gradually worse just the last few days), but there isn't the same wear pattern on the cleat. The float feels really tight, high friction, reduced range. 

Things I've done to try to fix it: I cleaned around the cleat, made sure the cleat bolts were tight, checked for contact between shoe and pedal (normal; consistent with good side), and examined for any cleat wear (none perceptible). I tried clipping the right shoe into the left side by hand, but it's hard to tell any difference when not actually on the bike. Update: I tried using some silicone lube - helped for my morning commute (~10 mi), but evening commute was back to bad.

Has anyone experienced abnormal cleat float resistance/friction/reduction? Could the shoe be contributing to this? Any ideas what I could check or change?
(This is more a collection of thoughts to help youtowards an answer, than an actual answer)

If wear at that point causes the shoe to sit a touch lower in the pedal, and the rubber sole to press against the metal, you'll get extra friction.  My shoes are wth me, and show a similar wear spot; perhaps on your new cleats it's just not yet obvious.  And that spot might actually contact something.  I could believe so from about 4:46 into this video (M324s, but the mechanism should be very similar) and also from my own shoes (used with M424s)

Because I ride on M424s comnbined with walkable shoes, I had to file bits off the pedal to stop them binding against the shoes.  To find where they were binding I dampened the sole and dusted the pedal with coloured chalk, then clipped the pedal (off the bike) to the shoe held between my knees.  I could then see where the chalk stuck to the shoe. You might try something similar, though it might be easier to see where the chalk (alternatively talc) rubs off the shoe.  You may not be able to make the problem go away with the file as your pedals don't have much material, but a light trim of the sole in the right region might help.

Which foot do you unclip more?  The one that shows the trouble?

You can also try throwing some light grease over all the metal parts.