Using Shimano Nexave ST-T400. Can select small and middle chainring but when trying to select large chainring, the selector lever moves without any 'feel' (almost like the cable is loose - but it isn't). Only at the very limit of its movement does it feel like it is moving the cable. The derailleur does move but only a very small amount and not enough to engage the front chainring. I have read the threads and watched the videos. H - L screws are all OK. cage clearance is fine. barrel adjuster is set according to videos etc. I am totally baffled. Anybody got any ideas?
Check for kinks, bends, or smashed sections in the cable housing. Also try putting something like tri-flow in the top end of the cable and the bottom end of the cable holding it upwards letting it drain deep into the housing. Put some tri-flow on the working parts of the Front derailler as well manually move it to see if it is "sticky".
Good maintenance to your Bike, can make it like the wheels are, true and smooth!
Can you make it work the top and middle if you adjust the cable. Or if you pull manually on the cable will the derailleur move the chain onto the big ring?
Ride hard or ride home alone!
Since your manual manipulation of the cable gets you into the big chain ring, it may 'just' be an issue with cable tension, though I'm not familiar with the shifter you're using. First check to see if you have way too much slack in the cable when you're in your smallest ring - you might be a full gear out of sync. You can try getting rid of all the slack with the barrel adjuster ('unscrew' the adjuster to increase the cable tension), or you can loosen the cable bolt at the front derailleur and fully re-adjust the cable that way.
Sorry if you've tried all this before. Make sure the shifter (and the derailleur) is all the way to the small ring when you're checking the slack in the cable.
I suspect you have a sticky pall inside the shifter. You say it moves freely at first and only seems to "grab" the cable after pushing pretty far. When you are in the middle ring and go to shift to the large, does the cable move at all as you push the shifter or does it only start moving once it feels like the shifter "grabs". Depending on what kind of shifter you have, you may be able to open it up and flush with chain lube to free up whatever is sticking. But sometime shifters just wear out.
But try to rule out any cable/limit screw adjustment issues first of course.