Error message in Words With Friends is preventing download? - "You already own this item."
You may have to contact the Words With Friends developers and request that they restore your purchases. They should restore automatically if you're logging into app with the same credentials however, some developers tend not to use this method so you have to buy the packs again. It's an easy way to make a few extra dollars. 

The fact you're getting the message that you've already purchased the pack is a good sign. Get a hold of the developers and see if there is anything they can do.