Next time you're trying to summon and don't mind wasting some extra time, see if the name you're about to summon is in your active list of players.

The most common cause of this sort of thing is trying to summon somebody that's already been summoned. If they're in your list of active players their sign will disappear immediately. If not, it can hang around a few minutes until you get an update "from the cloud".

It can be pretty frustrating if you're not actually connected to anybody with a sign down and you're in a high traffic area. Tons of junk signs from people that are no longer waiting to be summoned.

If you want to waste even more time, if you see a sign and it's not in your active list, try and find them in DSCM-Net and double click them to add them to your active players, and see if the sign sticks around.

Up to you if that's more or less frustrating than just waiting for a summoning failed, but if nothing else you'll be filling your active list with people that are actively trying to get summoned.