Yes, there are several conditions under which airlines may refuse a service animal trainer :
The animal violates safety requirements (too heavy, too large, etc.);
It may be a direct threat to the health or safety of others;
It cannot behave well on the plane;
In order for you not to have these problems you also need to provide a
DOT Service Animal Form In addition, a person with a service animal cannot be refused at the airport; if so, ADA assistance can be used to solve this problem.