Originally Posted by Trakhak

Gummy factory grease is the overwhelmingly likely explanation, as at least one poster noted above. That the shifter works more smoothly after (one) WD-40 treatment but still only clicks four times just means that the treatment barely affected the gummy-bear-like conditions in the lever's internal mechanism. It's like trying to dislodge gluey rice from the bottom of a saucepan by spraying a little water on it.

So your choices are to (I) remove the levers from the bars, pull the lever covers off, and soak the levers in several baths of WD-40 or (ii) do what I did when I was faced with similarly stubborn STI levers: spray a bunch of WD-40 in there, work the levers, and, after little or no improvement, give up and buy a set of MicroShift replacements.

MicroShift's levers employ a cruder mechanism with a much louder click, but it's obvious that they'll never gum up. Shimano's mechanism is undoubtedly an impressive engineering achievement, but I'll take crude and reliable over something whose maintenance requires the combined skills of a safecracker and a watchmaker.