Word from the top
This from the horse’s mouth (well, Albert at Madison’s mouth anyway)
It’s either the return spring or the internals. Internals can go sometimes. It’s rare but with so many moving parts these things happen. We stock spares and new internals should drop straight in. As with all Shimano spares they’re not cheap…
To give you some idea replacement Dura-Ace 7800 STI internals retail for £117.44srp (that’s per side). Ultegra 6600 come in @ £88.08srp
A replacement spring would be preferable @ £0.97srp!!!
What series and model is the shifter?