Possibly the simplest fix would be to allow each character to attack with either ability score, so that a paladin could use either Strength or Charisma to attack with any power.
Alternatively, you could tweak the restrictions slightly, as follows:
Ranger
A ranger can use Strength for ranged attacks with heavy thrown weapons.
A ranger can use Dexterity for melee attacks with light blades and weapons with the offhand property.
Cleric
All weapon keyword powers use Strength, and all implement keyword powers use Wisdom.
However, a cleric can use Wisdom for weapon keyword powers if he uses a weapon from the mace, hammer or staff group (alternatively, if he uses his deity's favored weapon).
A cleric can use Strength for implement keyword powers if he uses a weapon that can be used as a holy symbol implement (and you may allow his deity's favored weapon to be used as a holy symbol implement for such clerics).
Paladin
All weapon keyword powers use Strength, and all implement keyword powers use Charisma or Wisdom.
However, a paladin can use Charisma for weapon keyword powers if he uses a one-handed weapon.