The fullback has twice in recent weeks blanked Frank in public during and after games.
"Two situations where he’s very aware of the fact it didn’t look the best," said the Dane.
"We are dealing with young people. He’s 25 years old, with a big focus and pressure and sometimes you don’t do it in the perfect way and you should learn from it and do it better in the future."
On rumours of Brennan Johnson being sold in January, Frank also said: "I will not speak about Brennan, but any player that maybe does not play all the time will probably always have a thought about that, but we need more than 11 players.
"In general, I’m happy with Brennan. He’s training well. He’s a big part when we’re playing a lot of games and we will be playing a lot of games so it’s important to have enough to shoot with.”