Manual enrolments of teaching staff (e.g. co-teachers, teaching assistants, course viewers) in Moodle expire in 30 days (from the day you enrolled them), unless you change the enrolment period when you first enrol them.

So, when you enrol a member of your teaching team you should choose an appropriate duration or end date: How do I manually enrol my teaching team?

If your teaching team has reported loss of access to your module, click on ‘Participants’ from the left-hand sidebar and check whether their enrolments status is shown as Suspended: How do I reactivate suspended enrolments in Moodle?