The secondary PIX will reboot every 24 hours or so if it cannot see the primary.
First : check the licensing of the pix pair. Easy done with a show version command on both PIX.
Primary PIX need's to have a unrestricted (UR) license.
Secondary PIX has a failover (FO) license only.
Second : (If you use a failover cable, 6 feet long max.)
Check the failover cable. Since the pine-out are not the same on both cable end, the cable ends are identified "PRIMARY" at one end, and "SECONDARY" at the other end. Ensure that both end are inserted in the right machine.
Third : Both devices (primary and secondary) must have an identicale hardware and software version.
HTH
Mike