Your scenario is unclear. I do not understand "missing" the middle chainring. Are you saying that on a shift from the 55 the chain bypasses the 44 and heads directly to the granny? The common problem with front derailleurs is the chain missing the very lowest chainring and winding up wedged in the crook of the frame. Problems at the extremes (outer rings) are adjustment issues with the limit stops of the derailleur (usually). A problem with a middle ring sounds like bent or missing teeth. FWIW.

H