Is the bolt in up to the end of the thread?

Is it bottomed in the hole? Check another bolt.

Why isn't it loose now that the pressure from the manifold is off?

Put some penetrating oil(NOTwd40) on the bolt, and tap it on each side hard with a hammer. You are trying to get it to wiggle a bit and work the oil in, or strech the hole enough to loosen.

Keep wacking it till it gets loose. In the worst case you'll have to helicoil the hole.

John C.
Machinist