Could be a problem with cable routing on front derailleur... if the cable does not go over a little tab on top of the pinch bolt the effective lever arm will be shortened and require super high cable tension to operate; super high cable tension might be what you are fighting against to 'release' the internals to downshift on the front.
Also you might have the front derailleur set up so it is jammed up into the 'H' limit screw when in big ring, also causing high tension thatmust be overcome to release. To fix you must undo the cable and start from square 1 - set derailleur alignment (100% perfectly parallel with chainrings) and height (1 - 3 mm above big ring - no more).