From what I've heard of Texas soil on this forum, I'm amazed that
anything grows there! (just kidding....) Actually, in this case I think the soil may be the culprit. Magnolias like rich, loose soil, and if they can't have it they just won't grow fast. Even with a well prepared planting hole, the roots will quickly find their way to the clay barrier, and the
plant's grow will slow back down. You didn't say that your
tree seemed unhealthy, just that it wasn't
growing... It's the dirt.... move out of Texas!