We had a case in the shop recently of a bike that would sometimes refuse to upshift from the small to the large chainring. It was addressed first by tweaking the normal front derailleur adjustments. Twice. But, the bike returned, the problem was still there.
To change from the small to the big chainring with Shimano Road STI levers, you push the brake lever inwards (sideways). To change from the big to the small ring, you push a smaller lever (that nestles behind the brake lever) inwards. In a normal upshift, when the brake lever pivots in, the smaller lever behind it pivots in with it. If, though, you push the small lever a bit ahead of the big (brake) lever, then the shifter is being instructed to downshift and upshift at the same time, and it won’t shift.
I came up with the theory that, as a result of wearing warm winter gloves, the customer was sometimes inadvertently letting a finger slip behind the brake lever to pre-emptively push the smaller release lever to effectively abort the shift. I put my theory to the customer (Hi Roger!) who was, let us say, doubtful. Nevertheless, he allowed there was a slim chance that it made sense so off he rode home, pushing higher on the lever (with his fingers angled a little further forward) for front upshifts. We got a call an hour or two later from him – the gears were shifting perfectly.