Wild horse, the tail may be dull because of too much product. Try this: Only wash the tail with shampoo once in the spring and once again in mid summer. Get some 'ethnic' leave in hair conditioner (olive oil or similar) for humans from your local drug store. Apply this sparingly to the tips of the tail and massage a bit into the dock. Only maybe once a week! Groom the tail by hand, meaning comb through it with your fingers (this takes a while). Then use a natural fiber dandy brush to brush the tail, not a comb, not a wire brush. This also takes a bit longer, but the results will speak for themselves.
In short: Less product, less shampooing, more elbow grease!
You can see an image of my horse Paladin as an example what this can look like on the top left of this grooming tip article
Enjoy your horse!