Members don't stay logged in now Domain Mapping is enabled

Hi,
I've been using Domain Mapping on several sites and it's working great. An issue has just arisen though on a new install for a client which I've not seen before. We're using Wordpress 3.4, Latest release of buddypress and the latest version of Domain mapping on a multi-site wordpress network.

I've enabled multi-domain mapping also.

When members join and log in to http://www.modelflyermagazine.com all is well (all this applies to the admin superuser too).
Then if they navigate to one of the sub-sites such as http://www.rcelectricflyer.com they're no longer logged in, nor can they log in at that domain. The login link works, but upon logging and navigating back to the RCElectricFlyer.com domain they're no longer logged in.

Until I set up domain mapping everything worked perfectly and I've no clue what's causing it :slight_frown:

Has anyone else had this issue?

Thanks

Justin