This is a known bug (I believe this would be the correct thread for it, though there are a couple others now), so far as I can tell once you've added items to your HA in the wardrobe, you can't update the hair until you save the outfit in some way. The only workarounds I have are to either start with your hair on a naked HA THEN add clothes, click the "wear" button after adding your clothes to update the outfit on site and then add the hair and click the "wear" button to update again, or save your in progress outfit, load that outfit, and THEN add hair.
[font=times new roman]"There's no better vengeance than learning to enjoy again." [/font]
Hmm, I'm not sure, then, as my workarounds still do the trick for me (I can add hair to this HA in the wardrobe so long as I don't add any new items, and adding new hair to a saved outfit I loaded worked, too).
It could be browser-specific or related to add-ons, perhaps.
Or something else entirely, it's hard to say, honestly.
[font=times new roman]"There's no better vengeance than learning to enjoy again." [/font]