no free leases

[Jim Pazarena]

I have a pool of 40 IPs for transient connections.
I have "max-lease-time 600; # 10 minutes'
for my pool.

Yet, I frequently see no free leases available. I really do
not think that 40 transient computers are all connected
to my network at once; I tend to think instead that dhcpd
isn't re-using IPs after the 600 seconds I've set as the
max-lease-time.
Do these transients need to RELEASE their IP for dhcpd
to re-issue it? What happens if the transient shuts down
and simply disappears? Is there another command to enter into
dhcpd.conf to have it be more forceful in its re-issuing
of pool IPs?

thanks!