I appear to be running into a similar issue to that described here:
The AT&T DSL modem upstream of my OpenWRT device seems to take a while to be ready to delegate prefixes after the AT&T devivce reboots (an IPv6 address gets assigned to the OpenWRT device, but no prefix). When the OpenWRT device receives an IPv6 address but no prefix, it seems to interpret the failure to obtain a prefix as final, which then means that downstream devices can't get IPv6 addresses. If the OpenWRT device is rebooted or disconnected from the modem, it then resolicits for a prefix, and gets it (unless the AT&T modem went down at the same time, in which case the OpenWRT device is ready to ask for a prefix before the modem is ready to provide one).
Is there any way to configure odhcpd to retry prefix solicitation?