You can use the option "Send to repeated members" to send or not to send to repeated members.
The system will try to balance the number of sent items to a specific member. If a new member exists, that member will be selected first.
New members only have a few exchanges, and for each exchange they receive one slot. This means that there are not so many slots for new members and this slots are selected first and are used up quickly. Older and more active members have more slots available because they have more exchanges.