it sounds like you do not have enough spans in your ep curve.
Therefore, you can use an ep curve tool to create motion paths. However. the differnce with the ep curve tool is that you need to create more than 2 points. What I mean by 2 points is that you can't put one click at say coordinates 0, 0, 0 then at coordinates 0, 0, 4 and then say your done. This is because when you create 2 points, it does not create any spans. You can increase the number of spans with the rebuild curve tool. Just make sure you have more than 1 spans to be on the safe side, but do not ask me why you need spans in order to create a motion path because I do not know why. I just know that you need the spans in order for the motion path to work because I had the smae problem before.