I'm having some trouble with sewing curved edges. Sewing Machines Parts and Sales in Los Angeles

So I’ve started making dog bandanas for my shop but I’m running into some issues with the curved edge.
What I’m doing is sewing the V edges of two fabics together and then turning it inside out. I then sew the top opening closed.

The problem I’m having is that one fabric seems to buckle. Like there is too much fabric but I measure it carefully to make them exactly the same. I figure it must be a sewing issue. I don’t have any experience sewing curved edges. Do you have any tips or suggestions for me?