It's very common on these engines.
They could break and the threaded part stuck into the cyl. heads. I strongly advise you to take it to the dealer or to a shop who is expert and has the removal tools just in case.
If the threads will get damaged it will be a very expensive to repair.
There is a special method developed which is usually works, but not all the times, that's why you need the experts.
Please visit similar questions and answers on this subject by searching by Ford and your vehicle. You will find much content and feedback about plug replacement. And as Zee suggested...it may not be the cheapest thing, but having a trusted shop do the work is valuable.