You've explained enough - we can't diagnose further from afar. It's too much friction or some sort of interference. If the derailleur goes all the way to the low limit screw stop (where there's actually less spring tension) without being pushed it would seem logical that it's something to do with the middle position of the derailleur, or the corresponding position on the lever. To check the derailleur, click down to the small chainwheel position and then push the derailleur with your fingers up to the middle chainwheel. Back off pressure and note whether there is any resistance at all in the area of the middle chainwheel. If not then you're left with the shifters, or less likely - the cable, unless there's one isolated portion of the cable that affects only the middle chainwheel position - which seems unlikely. Flush and lube of the shifter may help - if not then you're down to replacement.