I ride with wide 700c tires, fenders, and a front platform rack on my touring bike, and have never had any trouble getting it to lock securely.

The one piece of advice I have, remember than some buses have 3 spots.

I was waiting at a bus stop, and there just happened to be another cyclist there. Luckily, the rack was empty, so we both were able to get on. The bus was fairly crowded, so I sat towards the back, where I couldn't see my bike. A few stops later, I saw someone's head as they loaded their bike onto the rack. I never panicked quite so much, I was convinced that someone had made off with my bike.