Goals from Jermaine Easter and Shane Lowry saw the Addicks suffer their ninth defeat at the Valley this term, the worst home record in the Championship.
Charlton have won eight games on the road compared to only four at home, and Powell was at a loss to explain the difference.
"I'm mystified by it to be honest. I'm baffled," he said. "I'm embarrassed for my players and for my supporters because we have tried everything with regards to performing at home.
"We had a good first half, we stood toe to toe and had a penalty claim that wasn't given and within a minute we were a goal down.
"Of course goals change games and we ended up chasing it. It was an outstanding goal by Shane Lowry and we ended up losing.
"We created the better chances but you have to take the ones that come your way.
"It didn't happen through good defending, good goalkeeping or us not hitting the target. We have just got to soldier on."