Hi all,
have a genuine first for me.
when giving the bike a clean, noticed that the chain wouldn’t drop into the two highest gears... I set about reindexing the gears, undid the cable, zero’d the shifter, cannot now get into the bottom 3 gears. (Not the limit screws as did work fine)
did this again, and again, making sure I followed the normal steps, no change.
it,s a 105 hydraulic shifter, only about 15 months old and has been well looked after. Mostly kept indoors, had regular service (by me) but don’t know why it’s not working???
any suggestions greatly appreciated

Is the rear derailleur hanger bent? By "the two highest gears", are you referring to the two physically smallest cogs, or the two physically largest cogs? If the former (which I understand is the conventional nomenclature), how did you manage to re-index the rear derailleur? The first step is setting the high limit, and the second step is setting the index, both of which involve the smaller end of the cassette.

Rear Derailleur Adjustment | Park Tool

Disconnect the cable from the derailleur. Pulling on the cable end with one hand, shift through the gears with the other. If you get the requisite number of clicks (number of cogs minus one, since you can't shift into the gear you're already in) with a corresponding change in cable tension, the shifter is not broken. Look elsewhere. Be grateful that every other component in the shift system is easier to fix or cheaper to replace.

STI levers are extremely reliable, although lemons do occasionally appear. I would assume the problem is elsewhere until you have eliminated all other possibilities.

If the cable is disconnected from the rear derailleur, can the derailleur be manually (pushed by hand) moved through all cogs? Also, as above, with the cable disconnected, pull on the cable with your hand to give it a bit of tension and see how many clicks you can get. If you get the correct number of clicks then the shifter is working, full stop.

You might want to check to see if your shift cable is frayed inside the shifter. This can cause shifting problems and if you don't replace it before it breaks it can be very difficult to get the old cable end out

Thanks all for your responses.

it turns out the cable had broken, took to a LBS and a replaced cable solved the problem, much cheaper then a new shifter.

apparently (not sure if 105 hydraulic shifters, or all shimmano hydraulic shifters) it’s fairly common according to the guy at said LBS.