"Log in with Ajax" plugin not working

We have a new website running on the domain we-love-pets.co.uk which is using the plugin to provide authentication to franchisees. It works intermittently refusing to connect some browsers and some computer/location combinations. We moved a copy of the site to pinkdev4.co.uk to investigate only to find that everything works perfectly. I have examined the servers closely and cannot find any differences apart from the DNS which could explain the situation. Can anyone suggest an area for investigation or a possible answer.

Richard M