Duck-killer is absolutely right. A modifed barrel, with a high-base 2-3/4 shell in #6 will take just about anything, from Turkeys to quail. I doubt you have any holes in your pattern. My extra advice is:
1. You should pattern your gun, becaue it is very often the case that based on the stock, and your shoulders, your pattern could be going high. I see this alot!
2. Are you sure they are in range? Crows are big birds, and as such, they always look closer than they are. Be sure you get them within 30 yards before you start to shoot, because by the time you take your lead and pull the trigger, they will be out at 40 yards, which approaches the far end of your range.
3. The modified choke is a good choke, because with modern plastic wads, it really shoots the nearly equivalent of a true full choke.
4. Hit the skeet range. and station no. 4. Shoot two full boxes at this station. It will help 'engrain' the lead into your brain.
Big Paulie