Skylanders Trap Team 2-Player Mode Difficulties (Wii U)

Lately I am having issues getting my son's game to allow a second player in Skylanders Trap Team (Wii U version). My controllers work (no low batteries or anything), are synced to the system properly (work in all other games), and have working nunchuks (also work in other games) and are properly attached before loading the game. If I'm in the home menu of the Wii U, the controller works, but when I load up the game, it disconnects and reconnects the controllers. The first one will light up/work, but the second will blink but not ever register as "2nd player". This happens regardless of which of my 4 controllers I use (2 are non-motion plus, 2 do have motion plus). If I try a second time changing the first to second, the one that clicks as "first" will work, the second will not. I cannot put another Skylander on the portal without "too many Skylanders on the portal" warning because the controller won't register itself as the second player beforehand.

I've tried exiting the game, restarting the game, restarting the whole system, I've switched nunchuks to other controllers - nothing seems to work. I recall having something similar happen with the Giants version, but exiting the game and going back in had always fixed it - now I'm stumped. Any ideas, anyone?

Thank you for any advice/suggestions,

A Stumped Skylander Mom