There are a plethora of synonyms for the phrase "was unable to remember," each with their own nuances and connotations. Some alternatives include "could not recall," "failed to recollect," "had no memory of," "did not retain," "was forgetful of," "couldn't retrieve," and "could not summon up." Depending on the context, some synonyms may be more appropriate than others. For example, "was forgetful of" implies ongoing difficulty with memory, while "could not summon up" suggests a more deliberate effort to remember that ultimately fails. Whatever the word choice, the sentiment remains the same: the person was unable to recall a particular memory or piece of information.