When clicking on citations in ResearchGate, this shows up when there is no citation from ResearchGate; this research isn't cited in any other publications. They search other publications for citation updates, probably not Google Scholar.
ResearchGate identifies citations with the help of automated algorithms. There is no way to add missing citations "by hand", also not for the staff. See also these discussions:
I saw Dr. Wolfgang R. Dick mentioned that ResearchGate identifies citations with the help of automated algorithms. I am struggling with transferring my citations from Google Scholar to Research Gate. Any Help?
Now, we all know about the issues regarding citations between Google Scholar and RG. We cannot fix that, but our responses can help them work on improving platform connections.