I have a lightly used Diamondback Axis XE that I've been having a minor front derailleur shifting problem. It is difficult to search for, I've read a few derailleur threads but nothing with these same symptoms.

Downshifting (going to big-little cog, this is a triple chainring) is fine, however upshifting (small to big) take pushing the shifter to its stop and holding it there until the shift completes. No chain rub or derailleur misalignment. Front derailleur is a Shimano TX50. Would this be a cable issue, or do I need to adjust the derailleur itself?