The resources that library has access to often come from aggregators. Aggregators don't publish things themselves, but instead licence material publishers. The licence sometimes includes access to full-text and sometimes it doesn't, so sometimes the full-text is available and sometimes only the title, abstract and a few other fields are available. So basically, it's a lucky dip; sometimes the full-text will be there and sometimes it won't. Sometimes access to the full-text can be delayed by the publishers, so that it become available after, usually, 6 months to a year.
Often, the abstract will give you all the information that you need, so don't get too caught up on the full-text.
Library resources will usually let you limit your search results so that you only see results where the full-text is available.
