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?