Was the parcel and population count sufficient enough to be upgraded to the fort level?
Remember that out of the 3 mayor titles, only 2 of those count as extras, and only the extras add parcels and pops. The main mayor title is consumed to claim the settlement as it is.
Also, each settlement size depend on a specific amount of parcels and population to qualify as such. A Fort requires 25 parcels and 75 pops.