Did this just start happening? Any chance you are working with smart previews?
For the most part Lr doesn't treat plug-ins as third party/external editors - plug-ins in Lr have their own dialogues and processes.
I was wondering if the OP was trying to use the Nik plug-ins on smart previews. Those plug-ins show up as external editors, which cannot be used on smart previews.
Aye, could be Jim - Topaz' "Fusion Express" enabler plug-in works as an external editor too.
Even so, most plug-ins don't (personally I think it's a stretch for Nik, Topaz and others to call these "plug-ins"), so it'll help to know which plug-in the OP is using.
As usual, the Devil's in the detail!