It’s possible you didn’t meet the conditions for auto enrolment before. For example, you might not have been earning the right amount to qualify.
Or, your employer might have postponed assessing you for auto enrolment. They’re allowed to do this for up to three months after you first qualify. After that they must enrol you immediately if you qualify. If you think you should have been enrolled sooner, you should raise this with your employer, see How does postponement for auto enrolment work?