Front derailleur and shifting.
I recently got a used Trek for a fairly good deal. It had Ultegra derailleurs on it, and it shifts fairly well for being an older model. So, I ran into a bit of an odd quirk, it shifts perfectly fine to the largest ring on the front derailleur if I have the bike lifted off the ground on a stand, but the moment I start riding it refuses to shift to that largest ring in the front derailleur. Everything else shifts fine, and the bike works great! So, I was wondering could it possibly be chain wear on the bicycle? I couldn't really find anything on this when I tried to look it up. I figured I might as well ask here. If you need any more information feel free to ask, and I will try and provide the information to the best of my ability. I am a bit new to road cycling, so bear with me!