minibarn

I have made a few hooded towels and added names to them. on 2 different occasions the thread for the last letter in the name starts coming out. The machine does several final stitches, which seems like it would lock the stitches in, but they don't. I have wondered if I should make sure I pull the top thread to the back and tye it off instead of just relying on that locking stitch. Hope this all makes sense. Anyway, can someone tell me what I might be doing wrong?

Thanks!