It's important to note here that lots of bad single-braid splicing advice is floating around. I believe Samson even changed their advice in their single-braid splicing manuals, after folks were seeing the old back-and-forth-then-bury approach fail.
To do it right, you need to pass the lines through each other so that they lock even with the tail not buried. (Sketch attached.) There is an obvious way to do this (passing the entire length of the line through itself to lock, and a topological trick way to do it with just one end of the line, but I can't describe that in writing.
Anyway, if you can't get your splice to lock as in the attached picture before burying the tail, I recommend you not splice single braid at all and use knots instead. If you are going to put a thimble in the eye, then a double overhand loop is incredibly secure. Otherwise, I'd use a bowline. Yes, the strength is reduced, but non-locked 'chinese finger trap' single braid eye splices are not reliable.
