Auto-complete feature for Texmaker citations

Made comment into answer which satisfied the OP

Options-->Configure Texmaker-->Editor--> Is completion checked on and add .bib extension \addbibresource{biblatex-examples.bib}. Check if you can see .bib file in Structure on left side pane or open the .bib file with Texmaker from File menu.

I found it out now after several tests. I have the following file for all bibliography stuff:

\renewcommand{\bibname} {C Literaturverzeichnis} 
\bibliographystyle{alphadin} 
\bibliography{bibtex/entries} 
\addcontentsline{toc}{section}{\bibname}

And I had the same issue like you. Everything is working and the bibliography and literature is correctly referenced. But I do not have any cite popup/sugguestion.

So the only thing I needed to do to always get it working, was opening the file with the commands above and one time hit compile. Now I always get the cite popups again.

I don't know to do that automatically, but at least it's working again now. Maybe someone could share how to do that?