Maelochs: Interesting to learn that most shimano derailleurs can handle a couple of teeth more than rated. That's good. Derailleur is not that old, maybe a year or two.
When I release the cable tension and move the derailleur by hand, it does drop down into the small cog OK. But I think I'll also check that again.

I might also consider the suggestion to open up the shifter and spray with WD-40.

Thanks!