It could be this:
Or, depending on what operating system you have and what camera you have, it could be something else.
If you are working on the latest version of MAC ( El Capitan ) then you would not be able to tether with Lightroom.
If it is another MAC version please let me know, also please check the following article for the known issue of tethered capture with Lightroom : Lightroom and El Capitan | Mac OS 10.11
no it's El Capitan , any idea when this will be resolved many thanks
Adobe has indicated that they have the updated SDK from Nikon, and that testing is positive. But there is no indication as to when the update will be available that will resolve the issue for you. Such announcements are never made.