HA pairing a Fortigate 100d

Hi All

I am trying setup HA on a fortigate 100d ,on either of the units when I go change the mode from standalond to HA it promptly loses the HA setting and goes back to the Stand Alone mode .

Has anyone come across this before ,the second unit has been factory reset so an issue some mention with dhcp being enabled on interfaces would not be applicable here .

Any assistance would be much appreciated.

Many Thanks

Clint