Why was a sign being sought?

In [Luke 11:29](https://www.biblegateway.com/passage/?search=luke+11%3A29&version=ESV) we have Jesus saying: > "...no sign will be given to [this generation] except the sign of Jonah." Yet Jesus had just cast out a demon. Did He say this because the people in His midst were "seeking from Him a sign from heaven" and He felt his present miracles should be sufficient? In that case, why weren't his current miracles enough? Jesus also mentions "your sons cast them out." Was this common practice in His day? Is that why the casting out of demons wasn't enough for the crowd in order to prove He was the Christ?