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I would guess that the bottom of your bowl has been distorted by coming into contact with the raising stake or has developed some distortion due to shrinkage of the surrounding metal. Simply place it on a flat surface and planish it down with a flat-faced or very shallow domed hammer - don't hit it too hard, you don't want to stretch the metal, you want to compress it. Work from the high spots to the low spots.

Rich

First you have to start with a flat bottom and keep it flat during the raising process. After a few rounds of raising you need to set the bottom. As Rich noted probably from hitting the stake. I use a round piece of wood (cut off baseball bat) in a vise and set the bowl over that. Then plannish from the out side in around the bottom. It may dome a little and then you flatten by holding over the wood.

www Metalrecipes -- heat and beat to the desired shape, repeat as necessary.
warren