sync failed between primary and secondary F5

I am fairly new with LTM's but we recently deployed an LTM 4000s pair in production and while we were testing in the lab we also had sync issues. Like Shaggy said first think to make sure that both your units are sync'ing with NTP. Something else that you can try (that worked for us) we broke the trust between both boxes, regenerate the device certificates then follow add the device trust to try the sync again (assuming the fact that vlan definitions etc are identical as in our case we had lowercase/upper case differences in vlan names between the boxes). Are these new boxes or sync has worked on them before?