You should add your legal name to your account, see "How do I add an alternative author name?" in https://help.researchgate.net/hc/en-us/articles/14292898263057-Updating-your-account-name for instructions. After this, it will be easier to claim publications that you published as Arthur Johnson. After this, search for the titles of the publications missing in your profile at https://www.researchgate.net/search.Search.html?query=&type=publication, open the pages of these publications and claim authorship (see also https://help.researchgate.net/hc/en-us/articles/14293005132305-How-to-add-research for instructions).