alternative host error

I work for a University. We use zoom for meetings, and I often schedule meetings for others, that I will not be attending. I am continually finding I am unable to add the attending colleagues as alternative hosts, even though we are all using university email addresses and their university provided zoom accounts, and the very same person's email worked just fine as alternate host the month or week before.  I reached out to zoom over support chat, and they said they weren't on the same license, and were using a free/basic account, which seems highly unlikely given our context, and my ability to add several of the offending accounts as hosts previously. How do i get this problem actually fixed???