In the first QB I played, my Shermans were hitting the German armor quite a bit but with no penetration. They hit so often but with such little effect that I thought the game didn't have the old "penetration" label anymore. Finally one round did some damage and I saw that all my previous shots were just bouncing off. After the mission ended I found out that they German tanks were Panthers and the results were probably historically sound.
In later missions I haven't had much trouble knocking out PZIVs or Marders. With the big cats I have had to learn to use bait, then strike from the side for kills - plus use lots of artillery.