A position created in the Volunteer Library has a set number of positions available. Once the number of registered users for that position reaches the maximum number of positions then that position will become inactive. Also note that a volunteer library position is available across all initiatives (events) and participants registered for said position from past events still count toward the maximum number of allowed spots for that position.
If you have only registered five individuals for the volunteer position in this years initiative and had twenty people registered for that position in last years initiative, the total count for that position would be twenty-five. It does not automatically go back down to zero participants for a new event, it encompasses every person registered for that position across all initiative.
If you see a volunteer library position go Inactive and you want to re-activate it edit that position and add more available slots and the position will become active again.
The other fix would be to create a new position with a similar name for each initiative and give this the set number of positions allowed for that particular event.