The head could be warped enough to cause the leaking. It could also be that the block is warped too. If the mating surfaces weren't scraped clean they could also cause leaks.
I don't think there is a way to fix this without removing the head. When replacing heads I always have them resurfaced and pressure tested to ensure they seal correctly (usually costs around $30). There is also a
spray on product you can buy for head gaskets that help them seal better.