Help with Canon lens EF-S 15-85 - Autofocusing

[mamandm]

Several months ago, I bought this lens as an upgrade to the 18-55 kit lens for my Canon T3i. The issue I have with the lens is the constant struggle to auto focus. Within the viewfinder, the focusing point is very small but I do try to place it on my subject where there is contrast. This morning, I gave up and switched to manual focus. Unfortunately, with glasses and outside light, I'm not the best a manual focusing through the viewfinder.

Is this just a quirk with this lens? If not, does anyone have suggestions?

Thanks so much!

(I am using a Canon T3i..I've tried in Program mode as well as Manual Mode.)

Mary