Sometimes brake hardware gets frozen due to the extreme heat they see, similar to exhaust bolts.
First try a good penetrating spray. Let it sit for a few minutes and try.
Make sure you are using the right tool. Since the bolt is stuck, I wouldn't use a ratchet to break it loose. Use a closed end wrench if space allows or even better yet, a socket with a breaker bar. You will be surprised by the additional leverage and force you will get with a breaker bar.
Hope this helps.