Incorrectly grooming, or brushing the tail too much, can cause hair loss which will make the tail appear thin and wispy.
Spray the tail with de-tangler. This will make it easier to brush and prevent too many hairs being pulled out.
Hold all of the tail in one hand.
With your fingers or the body/tail brush, release a few strands of hair at a time and groom through that section.
For any knots, carefully split the knot a few strands at a time with your fingers.
Continue releasing and grooming through the hair until the tail is completely brushed.
Do a final groom through the entire tail.