LyX: Blank box in available citations

Despite having entered a 'BibTeX Generated Bibliography' reading a bib-file placed in the same folder as the LyX files, I am - for some of my lyx documents - not able to access the references in that bib-file, when I want to insert the citation in LyX 2.1.1.

It is as if LyX does not 'realize' that the bibliography file has been called. The available citations box is left blank.

Does anyone know why/how this connection between calling the bibliography file and the available citations is lost?

Thanks!! Helene.