I noticed a couple times in Marleybone, that I have done some high damage spells that should do between 300-800 damage only doing 200 or less at times. This is even without any shields from the opponents. Is there a reason the spells don't work as well or am I just having bad luck.
Are you paying attention to the magic type of the monsters? As monsters become higher rank, they have higher resistance to their own kind of spell, and lower resistance (higher damage boost) when hit with their opposite kind of spell. So a Fire spell cast against a fire monster does less damage than it says on the card. The same fire spell cast against an ice monster does more damage than the card says.
If you are fighting a monster of your own magic type, either use a different school's spells, or cast your prism spell on them early (before they get their shield up) to convert the damage to something they are not resistant to. In Marleybone you can muddle through by buffing up your own school spells to overcome the resistance, but the monsters in Mooshu and Dragonspyre are so resistant to their own magic type that this strategy won't be enough.
The opposite pairs are: Fire/Ice, Myth/Storm, and Life/Death. Balance monsters take extra damage from spirit magic (life, death and myth).