Fluid leak or loss? Air bubbles in the system? Avid has a 46 page manual on these things you can down
load or www.park.com has a Repair section that covers some of this.

Pedal: Most such pedals have an allen key adjuster that varies spring tension. Try swapping cleats from
R<->L shoe and see if the problem switches with the cleat. Borrow someone else's shoe ditto. SPD
pedals are so common should be easy to find another shoe. Try your shoes on someone else's pedal.
Should be able to parse out some info that way. Cleat more likely the problem than pedal, depending
on the pedal. Some pedals are cheaper than replacement cleats.