What banana plug are you using?
The typical AR plugs from BB won't take 12 AWG. Most good after market plugs should handle 10 AWG with no problem. If you are using the later, you'll need to serve the stripped end of the wire - evenly wrap it in 24 AWG wire until it fits the plug of choice. Then tin the ends with solder so that the serving will not seperate fron the speaker wire.
Do Not fold the wire back on itself.