You had trouble most likely because of the way that Canon's AE Lock works by default.
You should be able to see in the viewfinder if the metering changes;
1st, the "*" will go away (it's down near the bottom of the viewfinder)
2nd, aperture/shutter speeds changing from the point you pressed * (AEL)
Canon's AElock by default will lock in an exposure, HOWEVER If no other button is pressed, the meter goes off after two or six seconds. If a picture is taken and you remove your finger from the shutter button, the meter goes off after two seconds (with most EOS DSLRs).
When the meter timer turns the camera’s meter off, you lose any previous AE Lock setting.
-To maintain an AE Lock reading after a shot is taken, you must either continually hold in the AE Lock button or be sure to put your finger halfway down on the shutter button within two seconds after a picture is taken. Otherwise, the meter timer turns off the meter and, again, any AE Lock reading is lost.
-To intentionally turn off an AE Lock reading with Canon’s traditional AE Lock system, you don’t simply press the AE Lock button a second time — that just takes a new reading of whatever you’re now viewing. Instead, you have to either be sure not to press any other buttons and wait six seconds for the meter timer to extinguish the meter system, or turn the camera off and then back on again.
This is pretty poor use of AE Lock IMHO, instead you should take advantage of the newish AE Lock "HOLD feature.
If you have a 5D3, you can use custom functions to turn on "AE LOCK with HOLD"
In this case, the AE Lock will remain on until you press * again. It's much more useful for the AE Lock fan.
