There's almost always a bit of trouble getting regen established. If it's not the critters eating them, it's competing vegetation overtopping them, and if it's not that, it's a bad frost or a dry summer that does them in. If you go the herbicide route, you have to decide: broadcast spray, or spot spray, and how much drift can you tolerate? Seems so simple, really -- plug the seedlings in and they should grow... but they often won't. The key here is patience. Your trees WILL establish, given time... like 10, 20, maybe 50 years? You can help them along some, but they are the ones who end up doing the growing. There are really two major milestones. The first is "free to grow" -- that is, the seedling is established and will outcompete brush. It'll usually be about 4 feet tall when this happens, and further herbicide treatments or other brush control will have no effect. After that, it's on auto-pilot for awhile. You might think about a pre-commercial thin in the next few years. The second is a height of about 30 feet; this is where the tree has grown past silvicultural treatments and is subject solely to microsite factors. At that point, thinning will primarily affect volume growth rather than height growth.