Wait. The message means that
that particular ward is full. Not that
all wards in that city are full. So if you're in the Battlecraft Ward, and you get that message, you can still try summoning your retainer into the Ironmongers Ward, the Tradecraft Ward, the Chirurgeons Ward, etc.
I suppose it's possible that every ward in Uldah is full on the Lindblum server. If that's the case, congratulations on having a super busy server.

Otherwise, I'd suggest trying to summon your retainer into another ward in Uldah.