It seems that new sites are easily sandboxed now... completely disappear off radar, but still indexed. Building more links always fixes the problem immediately. Being in sandbox is fine, although its a bit depressing. It's like being on probation, and you need to keep building links (any links) to get it out of sandbox.
I got a single quality PR5 backlink to a relatively new site....sandboxed, no where to be found! I waited 2 weeks, still nothing. I got fed up, so ordered 50 high PR blog manual comments (on actual PR3+ pages, but super high obl ...very spammy), and the next day site was ranking better than ever! I've had site sandboxed after bookmarking, and then recover after high PR forum links!
So, my advice if you are sandboxed, is to order a small package of manual high PR blog or forum, or some other type of high PR, low quantity backlink, just to keep link velocity ticking along, and should see some improvements