Sound check issues on 7th gen nano

I have Sound Check turned on in iTunes and on my 7th gen. nano. My songs all play at about the same level in iTunes, but on my nano, some songs start out at a higher (or lower) volume for a few seconds before leveling, and some songs play at a level that's too high (or low) the whole way through.

All songs are MP3s without DRM. iTunes is up to date, as is the nano.

Any thoughts, ideas as to what might help?