Custom Heads Won't Work?

Hello,

I am making a new spawn where I use heads for server ranks. (As in my head for Owner).

But I will need new Helpers and Moderators for the server where I was wanting to use the question mark head. I tried both /skull MHF_Question and the Microblocks Plugin, but neither one worked. However I think every other head works, I've tried arrows, animals and some blocks.

Is anyone else having this problem, and is their a fix?

Thanks

 

The problem might possibly be the version of minecraft you're using. Whenever I use player heads on 1.7.2 they don't work (although sometimes, rarely, when I'm looking at just one head, it works). Using 1.7.10 however, does work for me.

If you're looking for more heads I found this to be godsend:
http://www.planetminecraft.com/project/decoration-heads/