@Rose81689 If you had more letters received then sent there might be a break in receiving new mail until the numbers get closer. You now have one incoming letter and several available slots that will be used by other members to send you mail.
@OrigamiGrace You have selected to send/receive only from US. This is okay but it also limits the number of available members. There are about 7 members available from US now. If you can't get their addresses, probably you have already sent them a few letters and they could have choosed not to receive again from you. Or maybe you have blocked some of them. Check the "Blocked members".
@naniani I'm studying the posibility of reducing the time for auto registering.