If it's anything like what some of my downstreams have encountered, you'll want to explore the options to set communities on your outbound announcements to 6939 and/or 3356. Most likely, as 3356 announces your routes to its peers, those peers are assigning a peer-level local preference. Meanwhile, as 6939 announces your routes to their upstreams, those networks are using a default or "paid customer" local preference. Since local preference comes before AS path length, you have to remotely twiddle local preference to get results.
HTH,
Pete Templin
Senior Staff Engineer
TexLink Communications
(210) 892-4183
pete.templin@texlink.com