It's very, very hard to do 20 years of service. Individuals typically have very little choice in staying. They must be promoted in a timely fashion or be booted. The problem, at least with the Navy, is that the higher enlisted ranks have fewer openings, meaning more competition, and seniority becomes a factor in promotion. So you may be a great soldier, get good marks from command, and score well on the exams, but still get passed over because people four or five years ahead of you get the job instead.

Each year that you miss a promotion compounds. You can only miss so many promotions before it becomes impossible to hit the rank where you may retire.