Using the CiviCRM menu, if you navigate to Administer > CiviMember > Membership Status Rules (
civicrm/admin/member/membershipStatus) you should see a list of membership status rules. These dictate the "workflow" of a membership, and they need to chain together correctly. For example, on the sample site I have handy the rules are:
- New - ends +3 months from "member since" start date (configuration pictured below).
- Current (rest of membership - doesn't have start/end config in my sample site, but is the "default" state)
- Grace - starts end of membership, ends +1 month from that date
- Expired - starts +1 month from end of membership
- Pending, Cancelled, Deceased - not part of the automatic process.
Here's what "New" looks like.
I suggest you inspect the dates those members are trying to renew and work out where in this workflow they would fall given the rules on your site (expired or grace presumably, since you seem to have a fixed year membership and it's now May in the next year). It may be that a membership state has been removed from the workflow and this leaves a gap, or that the timeline has been adjusted for one but not another.
Just a guess, that's where I'd look first!