In ResearchGate, citations are identified by an automated algorithm. This algorithm does not work ideally, i.e. it does not find all citations or wrongly identifies others. It is not possible to add citations manually. The only way for users to have an influence this process is to upload full texts of papers (as far as allowed by copyright rules). See also this method develeoped by Andrejs Krauklis : https://www.researchgate.net/post/Method_for_Improving_Recognition_of_Citations_References_on_ResearchGate/1
Here are some of the discussions on missing citations in RG:
https://www.researchgate.net/post/RG-citations-how-are-they-calculated/1
https://www.researchgate.net/post/missing_citation_number/1
https://www.researchgate.net/post/Why_are_some_of_my_citations_not_shown2/1
https://www.researchgate.net/post/Why_the_citation_is_not_shown/1
https://www.researchgate.net/post/How_can_we_add_citation_on_research_gate/1
https://www.researchgate.net/post/How_add_references/1
https://www.researchgate.net/post/Citations_from_RG/1
https://www.researchgate.net/post/citations_in_RG/1
https://www.researchgate.net/post/How_do_you_fix_missing_citations/1
https://www.researchgate.net/post/How_to_add_missing_citation/1
https://www.researchgate.net/post/What_if_I_have_citations_in_the_research_gate_indexed_papers_that_research_gate_does_not_show_them/1
And here are several discussions on how to increase citations in general:
https://www.researchgate.net/post/How_to_increase_the_citations_to_my_published_research_papers/1
https://www.researchgate.net/post/How_do_I_increase_the_number_of_citations_for_my_paper/1
https://www.researchgate.net/post/How_I_can_increase_cite_my_articles/1
https://www.researchgate.net/post/How-to-increase-my-citations/1
https://www.researchgate.net/post/How_to_increase_the_citations_of_your_sole-author_or_multiple-authors_papers_published_at_the_Renown_Journal_in_the_field_of_your_research/1
https://www.researchgate.net/post/How_the_citations_can_be_improved_of_research_papers/1
https://www.researchgate.net/post/What_are_the_methods_to_increase_the_number_of_citations_of_my_articles/1
https://www.researchgate.net/post/How_to_increase_citation_of_article/1
https://www.researchgate.net/post/How_can_my_article_citations_increase_from_zero_to_hero/1
https://www.researchgate.net/post/How_Citation_can_be_increased_in_view_of_h-Statistics_for_a_Scholarly_article/1
https://www.researchgate.net/post/How_to_increase_citation_of_an_article/1
https://www.researchgate.net/post/How_do_i_increase_citation_of_my_research/1
https://www.researchgate.net/post/How_important_are_citations_Are_they_overrated/1
https://www.researchgate.net/post/The_power_of_keywords_and_audience-centric_papers_Reflections_and_thoughts_on_number_of_citations_publishing_and_highly-impacted_journals/1
https://www.researchgate.net/post/Funny_Article_Titles_to_increase_citation/1