There are many possibilities that an issue like this may happen. I recommend you to check the links that are mentioned in Wolfgang R. Dick's answer.

The question about missing citations in Google Scholars and other platforms has been discussed many times. Some of the answers may help you:

https://www.researchgate.net/post/missing_citation_number

https://www.researchgate.net/post/How_can_we_add_citation_on_research_gate

https://www.researchgate.net/post/How_to_include_all_citations_on_RG_and_Google_scholar_citing_your_manuscript

https://www.researchgate.net/post/How_to_add_missing_citations_on_Google_scholar_Research_gate_and_Scopus

https://www.researchgate.net/post/Why_are_some_of_my_citations_not_shown2

https://www.researchgate.net/post/How_add_references

https://www.researchgate.net/post/Does_anybody_know_how_we_can_add_the_missing_citations_to_our_profile_in_Google_Scholar

https://www.researchgate.net/post/Does_anybody_know_how_we_can_add_the_missing_citations_in_Research_Gate_as_will_as_Google_Scholar_profile

https://www.researchgate.net/post/Does_anybody_know_how_we_can_add_the_missing_citations_to_our_profile_in_Scopus

https://www.researchgate.net/post/Does_any_know_how_can_I_add_missing_citations_to_google_scholar_and_resaerch_gate

https://www.researchgate.net/post/Is_there_any_method_to_add_missing_citation_of_your_work_in_RG_As_most_of_the_papers_have_more_citations_as_per_google_scholer_but_very_low_in_RG

https://www.researchgate.net/post/Is_there_any_to_add_the_missing_citations_in_Google_Scholar_Profile

https://www.researchgate.net/post/How_can_i_add_missing_citation_to_my_Google_Scholar_profile

https://www.researchgate.net/post/How_can_we_add_citations_in_google_scholar_and_Research_Gate_for_our_articles

https://www.researchgate.net/post/How_to_add_missing_citations_on_Google_Scholar

https://www.researchgate.net/post/Citations_from_RG

https://www.researchgate.net/post/Method_for_Improving_Recognition_of_Citations_References_on_ResearchGate

https://www.researchgate.net/post/Can_someone_help_me_understand_why_there_a_difference_in_the_number_of_citations_shown_in_a_persons_Researchgate_and_Google_Scholar_account

https://www.researchgate.net/post/Why_is_citation_count_of_research_articles_different_in_Research_gate_from_that_of_google_scholar

https://www.researchgate.net/post/citations_in_RG

https://www.researchgate.net/post/How_do_you_fix_missing_citations

There are many possibilities that an issue like this may happen. I recommend you to check the links that are mentioned in Wolfgang R. Dick's answer.

Isra if you check in your account profile at google scholar there are ways you can add an article. If when you search with Scholar and you find the article then it is easy to claim that article as your own. And if not you can still manually add this paper to your profile

Hope this helps