The problem may be caused by an error in the article citation metadata. If you have landed on the correct publisher's site, look for a search box and copy in a few of the article keywords, then search anew.
If this doesn't provide the correct article, look for a link on the journal homepage that may say something like "All issues" or "Archive", then drill down to the specific volume and issue that the article should be in. Sometimes, an article is no longer available; in this case, you can submit an interlibrary loan request for the article you require.
Another special case is when your citation is for a poster or presentation from a set of conference proceedings. Many conferences do not provide the full-text for presentations, and will provide the abstract only. Sometimes the individual abstract does not have its own page on the journal website; instead, if you use volume, issue, and page number information, you can find the abstract included along with a complete set of other abstracts as one full-text/PDF document.
