Whenever I spawn in an ender dragon, it only justs flaps its wings and does nothing. Can you help me?

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Whenever I spawn in an ender dragon, it only just flaps its wings and does nothing. Can you help me?
What I expected to happen was...:
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I thought the ender dragon was going to destroy my world, but it didn't
Describe what happened here

Steps to Reproduce:

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2. I summon it and it wont work
3. ...