Speedplay pedals coming unclipped

Due to an injury I've not ridden much this season and have just recently been given the green light by my doctor to train full-tilt again.  I've been coming unclipped out of my Speedplays a lot.  I suspect it's just sloppy/careless form but can't help but wonder if the spring on the cleat ever gets weak and makes the pedals come unclipped too easily?  As far as I can tell there are no adjustments on these...not sure what the name of them are but they are the old style pedals without the divets around the pedal. 

~Mike