There are several synonyms for the phrase "not register", which refers to something not being recorded or noted. Some common alternatives include "not recorded", "unrecorded", "unaccounted for", "missing", and "overlooked". Other possible synonyms include "unrecognized", "ignored", "disregarded", "forgotten", and "omitted". The context in which these terms are used can vary, ranging from accounting and inventory management to social situations where someone may not have been officially acknowledged or considered. Regardless of the context, however, these synonyms all convey the same basic meaning of something not being documented or recognized in some way.