I'm... not sure. If you are dispatching your retainer from the bell, it may be that some sort of server processes is bugging up (the system appears to have been designed expecting the wards to almost never be full) - I don't know for sure since I don't use the bell like that. Mine just sit in the Chirurgeon's and Ironmonger's wards permanently and I go to them.

If you summon your retainer with the bell, the game transports them away from the market wards and to your location for item management. Thus, their items are no longer available for purchase until they set shop back up. This is done either by going to the desired ward and summoning them there, or by dispatching them with the bell-menu. It is generally not advised to use the bell to summon your retainer if they're selling in Ul'dah since you could lose your spot. Go to the ward instead.

In your circumstances, I'd suggest using the retainer list to check your retainer's location and going to that ward to see if they are actually there.