DHCP hosts not reliably propagated to DNS

I recently set up a new filewall with IPFire, but sometimes hosts that pull a DHCP address don’t get propagated into the DNS server properly. When this happens, if I log into the firewall and do this:

# /etc/init.d/dhcp restart

…then the host will show up in DNS.

My suspicion is that whatever mechanism that the unbound-dhcp-leases-bridge process uses to tell when it should regenerate the table is not working reliably. What can I do to help troubleshoot this?