Hello, and welcome to the forum.
You can check out the official Wall breaker FAQ here:
http://forum.supercell.net/showthrea...d-how-it-works
Or you can just read from this quote:
Regarding your issue, the reason why they acted that way is because the buildings that are at the first row, behind the walls, are still considered "enclosed" by the wall breakers, thus leading them to breaking it.
Few methods to overcome this issue is to
1) Clear off buildings on the first layer first before moving on.
2) Attack from the corners (Situational, depends on the base design)
3) Sending excessive wall breakers (Situational, might get shot down before reaching the 2nd layer)
Here are 2 nice tutorial vid for you to learn from
Video Link 1: Wolfz's Guide to New Wall Breakers AI in Clash of Clans
http://www.youtube.com/watch?feature...&v=YXieLZ3y3TA
Video Link 2: Flammy's ULTIMATE Wall Breaker Guide
http://www.youtube.com/watch?v=jEjOSEN9Llo&list=PLmcCn9jl33OUSpLifOBYh65z 7kWJntB_V
Hope this helps.
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Edit:
Flammy is not a mod.