Are you taking it out and rebraiding it weekly or braiding and leaving it for good? I've had the most success with washing my geldings tail in late fall, soaking it in MTG, and braiding and bagging it. I then unbraided once a week, reconditioned it with the MTG, combed it out, and braided again. I did this every week until spring. That worked great for me in the past, although I didn't do it this year because I was lazy. You can't braid it once and leave it for months, this will cause the hair to become dry and eventually break it.
But here's my bad braiding experience: I've been growing Chance's mane out for over a year. It finally got down to just past his neck. I gave him a bath over the weekend, washed and conditioned his mane, and braided it in 6 larges chunky braids. I turned him out with his new pasturemate, left, and when I came back his pasturemate had eaten half of a braid! He now has a choppy spot in his mane that's about 3 inches shorter than the rest. Ugh.
I've spent most of my life riding horses. The rest I've just wasted.