Originally Posted by Taon

Since this has come up a few times, I'd like to add this to the compendium of info for beginners:

Exchange tickets cannot be saved until the next unit pool update (typically when a new event begins and the featured girls from the previous event are added to the general pool), and sometimes expire well before the pool update.

DO NOT TRY TO SAVE EXCHANGE TICKETS.

Its very unfortunate how many people report this issue every single time there is a ticket on sale.