What kind of camera are you using? Is it digital? See if you have something like Center-Spot under Exposure Metering and something like Center-Zone for your Focus Zone. That should make your camera focus on the object in the center.

Also make sure you're camera is a the proper length from your subject so that it can focus properly.

Yes the focus has caught the cactus but the flower was too close to he camera. If you need to focus closer try holding a magnifying glass in front of the lens.