Is the "inner part" of the bow, or the shorter part, the starting row? It's really, really easy to accidentally pull the original chain too tight, or for rows to get a little looser when you get past the chain. If you started by weaving in a tail that's several inches long (always a good idea) then you should be able to bring that back out and work a little slack back into the base row.
There are a couple of reasons. If it is looking like a U then your chain is looser than the stitches. You might want to use a smaller hook for the chain. If a Rainbow shape your chain might be to tight and you want to use a larger hook for the chain and than just go a size up for the stitches. If you only go through one loop on the chain instead of 2 your also at a higher risk of it bowing. Hope this helps.