OK. Then in all likelihood you have something on one or more slides in your PPT deck that is causing a conflict with Captivate. The best way to find it is via a process of elimination.
If you at least get to the point where Captivate shows you the list of slides and you have the opportunity to select which ones you import, then just select small groups of slides (e.g. 5 or 10) at a time and see whether you can import at least some of them without issue. If you can with some but not others, then you can narrow down where the problem slide or object is located.
If you cannot get that far in Captivate, do the breakup in PPT itself, copying slides from the problem deck into another blank PPT file and try importing that.
Bear in mind however, that even when you DO find the issue and remove it, all you end up getting with bringing PPT files into Captivate is a lot of static slide backgrounds. You won't get all your animation and audio etc. So in most cases you are probably better off rebuilding again in Captivate from the ground up.