Working with all types of Deer hair is a skill unto itself and entire books have been written on it.
As far as bucktail goes what the others have said is valid and all could be the root of your problem, hopefully I can share some others.
Not only are some bucktails easier to work with than others but parts of the bucktail also provide better stuff to work with. As a rule you'll find the heavier coarser hair toward the base more difficult to tie in without flairing than the stuff towards the top which tends to be less hollow.
The issue of flaring is caused by the the deer fibres being hollow. The coarser the fibre the more hollow it is, when the flytying thread is tightened against it the walls collapse the two sides compress together causing it to flare out. By using the finer fibres at the upper end of the bucktail this is alleviated.
It can also be corrected by tying in with tight turns and then a few firm, but not as tight, turns of thread are made to help it retAIn it's shape. A good drop of flytying cement at this point will keep it all in tact so it remains tuff but does noet unravel.
Also, when tying decievers do not try to tie the bucktail at the head oin one lump around the shank. It should be done in three stages. Tie in a clump on the far side of the hook, another then on the near anf finally one on top. By doing it this way ytou get better shape, a more realistic profile and have less chance of flaring because you are tying it in with smaller clumps.
Now just add eyes and epoxy.
I am flat out doing some commercial tying at the moment but if I get a break tonight I'll try to post some close up pictures of ways to make the tying wraps without flaring the hair, failing that, it'll be a couple of days.... good luck.