Domain Mapping Plugin is not allowing any more domains to be mapped

I've been running a Multisite site setup on a server for about 6 months with the help of your awesome plugins. The problem just appears to be as simple as a button missing to add more mapped domains to a site within the MS setup.
1). External domains have the A record pointing to my permanent IP address
2). I can add the domains to the Mult-Domains plugin
3). When I navigate to the site where I want to point the domain, under the Tools > Domain Mapping option I have no button/option to add a mapped domain.
4). I checked all my sites and this is the problem on all of them. Some already have mapped domains linked to them and the sites are externally accessible.

Any ideas why I can not add this? Would a screen shot help?

The only thing I can think of is that one of the updates has stopped this functionality working.