I have a Canon EOS T3i. Last night we had a lightning storm and I put my camera on a tripod, set my shutter priority to 2 min exposure. The shutter just would not trip! What am I doing wrong?
Based on your description, my best guess is that you had it set for 2 seconds rather than 2 minutes, since the shutter priority mode on most Canon cameras won't go past 30 second exposures, including high end professional models. I'd also hazard that you had it on Auto Focus with it set such that it won't take the photo until focus is achieved. Since it was no doubt very dark, focus probably could not be confirmed and thus the shutter would not activate.
In order to accomplish your goal, you will need to use manual focus so that the shutter isn't delayed and then use an intervolometer in Bulb mode to keep the shutter open for a longer time.
It is also worth noting that a 2 minute exposure may not be the ideal way to capture what you are trying to capture. At 2 minute exposures, you will a) have a lot of noise from the long exposure and b) the flash of lightning will be a relatively small portion of the exposure. You may be better off to take multiple shorter exposures and use whichever exposure happens to catch lightening that is of interest to you and/or stack multiple such shots after the fact using image stacking.