Sometimes the dog is perceived as not the quality to attain the owners goals. Sometimes the dog just is not drivy, intense, confident etc.
Sometimes the club/handler/style just does not fit the dog.
A lady in Canada bought a young dog off of me....returned him in 3-4 weeks because he did not "fit" the training program. He was possessive of the toy/reward and did not automatically re-engage....he needed to be taught "two ball".
He went to another experienced handler who talked to the helper he was working with....he is about 6x IPO3, with Vs in all three phases (a few weeks ago was 97-95-97 I think, HIT) shown as a 2 and 3 at National levels, with a top 10 at a National...Get updates occasionally from the first buyer's coach/trainer who loved the dog...and who is friends with the owners coach....the first buyer still has not done anything significant and has gone through a couple of dogs.
So it is often a very very good dog who "washes out" because of human error or impatience.
Csabre Sch1, Hexe Sch2, SG Bengal, Lynx, Komet, Kira HGH & Kougar v Wolfstraum ~ Ziberia IPO1 ~ ATB Basha, Kougar & Kyra, Fenja, Sch3s, Cito, Sch2, Danger RH & Alice Sch1s ~Kelsey