This happens from time to time depending on the activity on the club. There is a system to compensate for the lost exchanges and for the increase in members. Everybody receives an extra slot proportionally with the number of positive sent exchanges.
I try to keep this in balance. If I increase this more, you will be sending more but you will send to members that are not as fast as you to send back and you will be waiting longer to receive as many lots as you sent.
New slots are open every time an exchange is registered. You can check from time to time, new slots will be available.