cyclogic

I have a nice new Giant roadbike that I bought new about 9 months ago, complete with Shimano 105 shifters. After about 6 months, when riding home one night and changing from middle to big chainring, the shifter let out a loud crack, and refused to change. Basically it would shift from the small to middle ring OK, but wouldn't go further. Although I don't know what these thinks look like inside, but I would imagine one of the teeth had broken off the ratchet inside the shifter, as there was no resistence on the lever when trying to shift up. I took it back to the store and they replaced it under warranty. To be safe, I cleaned out all the cables and lubricated everything to make sure it wasn't undue pressure required to change gear.

Riding home two nights ago, the same thing happened in virtually the same place.

Am I just unlucky with these levers, am I doing something wrong (this is the first bike I've had with shifters on the brake levers), or are they just really prone for this sort of thing happening?

Although when changing gear, the lever needs a bit of pressure, I'd be disappointed if it really does break that easily...

Any ideas?

Thanks in advance.