The T clamp is finicky. You need to make sure that you are in the clamp by opening it up all the way and holding the wire near the screw when tightening. If you think the wire needs to be thicker, strip a bit more than twice the clamp depth, fold it, and twist the folded wires into a tight twist. That should bulk them up nicely.
Ironically i was going to post about this yesterday and ran out of time. I took a pic of the T clamp wire bar with its divot in the centre. The divot doesn't work for wire gauge smaller than 12. I found the same issue in the past couple of days as i finally got the new surrounds off of boxes and onto the walls. I was hoping for a bit more direction with the use of the T clamps in the speaker manual. I have 14ga wire and tried the double back method that Trevor mentioned but it wasn't always successful. I found the best way was as rrlev describes and to keep the wire pushed to one side closest to one of the screws while tightening down (DO NOT OVERTIGHTEN OR THE SCREW WILL STRIP).
Worst case you can still use the binding posts on the back if something happens with the T clamp screws.
