I have set two points for the opacity, but the text won't fade in. It just appears with no fade. I can see the opacity level changing, but can't visually see it.
Nobody can tell you anything without some serious info. You have not provided anything useful, beginning with your version of AE and a screenshot of the timeline.
Which opacity did you animate. If you used a text animator then that is a different discussion. To figure out what is going on with the text layer select it and press the U key twice to show all modified properties. That should point you in the right direction. If it doesn't please post a screenshot and a detailed description of your workflow.