I have these pedals: http://www.cyclesurgery.com/shimano-a530-spd-pedal/pedals-road/bikes-components-bikewear/fcp-product/19163?listing=true

Lately though, i've found it increasingly difficult to clip in and out, but only on one side. When i'm out and about my right foot stays clipped in pretty much the whole time, and my left foot is the one I use for clipping in and out when I need to stop at traffic lights etc.

I have no problem with the right pedal, but on longer rides the left pedal will start off fine, but 'deteriorate' over the course of the ride. Why would this be?

It's not the pedal tension as before I go out I set them both the same (usually quite loose) setting. Is there some kind of pedal maintenance that I should be doing?