Well, if you have the mana, you must not have had the pips to cast the spells that came up. How many pips did you have at the time? What cards were in your deck when this happens? All high attack cards coming up is rare, but it does happen.
Did this just happen? Did you recently add new cards?
Although you may have enough mana, you also have to have the minimum number of pips that each card requires. If you don't have enough, then the card would be grayed out.
So, for example, if you put all high-level Treasure cards in your deck that require 4 pips each, you won't be able to use any of those cards until you've gotten 4 pips during combat (and, of course, each card would use up 4 pips).
The pip requirement is in the upper left corner of each card.
