Are there any premixed fertilizers in the soil? They can be pretty devastating to seedlings sometimes, since you never really know when they're going to split open and deliver their goods. The soil may be good quality, but it doesn't look like it's aerated all that well. A 30-50% mix of perlite would help the roots 'breathe' a lot easier.

Also, a 400W HPS may be a little intense for such young seedlings... unless you're able to keep it real close, and not burn them. Because of the need to keep the light further away, it may cause them to stretch a good bit early on. Fluorescents are great alternatives for seedlings and clones.