Full text is not always available for every article or for every year of a journal that we subscribe to. It may be that our subscription began with later issues, or ended earlier than the year you need; it may be that the database arrangement with the publisher did not include all years, or that contract terms have changed recently.
Look carefully at the available years in BU Libraries Search results - sometimes we have overlapping suppliers of titles and there is another online access link that has the issue you need.