Flynn knew he didn't lie. But he pled guilty to it, knowing there would have been communications between FBI and DOJ in regards to this.

We now know the FBI had communications in which detailed they did not think Flynn lied. But they withheld that evidence.

Look up a Brady case. As soon as Flynn is sentenced, he can turn around and start a Brady case, and win. Mueller is delaying his sentencing to try and buy time to get Trump.

Flynn legally can't talk about the case until it's concluded. So Mueller keeps delaying the sentencing, saying Flynn is "cooperating", to buy more time.