Failing to claim everything the applicant is entitled to
On points-tested visas, applicants regularly leave out points they qualify for, such as partner skills or regional nomination. On sponsored visas the equivalent is failing to evidence the specialised nature of the work or the employer’s commercial need, which is the single most common reason a subclass 400 is refused.
A well-prepared application is not longer; it is consistent, complete and lodged with the right subclass. That is most of what separates a smooth grant from a refusal.