I had the exact same problem except with Trigger Happy. Took the package back and got home and found the new one had a defective portal that only would read different Skylanders in different spots of the surface.
Ended up asking for the portal from my first package swapped with the defective one and now all is good with all 16 of my Skylanders.
My girlfriend and I finally had a chance to sit down and play it and we were pretty disappointed to find that Sonic Boom would only work if there was already another skylander in action. It's not a gigantic deal but it's just odd.
All of the 7 other Skylanders work just fine but for Sonic we need to place a sheet or poster over the portal and have him "hover" for it to get a reading.
I think the 360 version is having a tougher launch than the other platforms for some reason. Even the reviewer on Destructoid commented on issues with the portal for some monsters, and it mentioned he was reviewing the 360 copy.
I got it PS3, and while I was pissed about having a battery-dependent portal, it's been flawless so far. Fingers crossed...
Captain Anderson also has his picture on his desk. Kaidan is such a slut. - khrulez