Nest built but no visits for over a week..

Should we be worried ? The blue tits have used same nest box for the last 2 years and last year we had 9 fledglings and all survived They were a bit late starting this year and seemed a bit half hearted building the nest in fits and starts Looks pretty much finished now but no more activity for 9 days  apart from a blue tit looking in briefly 2 days ago Has anyone else come across this ? 
Many thanks 

Bob