More information is needed here.
1. What Operating System (OS) are you using?
2. Version of Firefox or other browser e.g. SeaMonkey are you using.
3. What version of Adblock plus (if not the
latest).
4. Which (if any)
filtersets are you using?
5. Page in question could also be helpful.
Off the top of my head:
1.
Elements are not currently being showed in blocked items list. This could be the cause.
2. Is it possible any other applications you are using is blocking some content? Test by opening the page in another browser and see if you can see the content. Firewalls are a very commonly the cause if adblock plus isn't blocking something.