tracing PDF

When I lay a PDF into my 8.1, and I zoom into trace, I need to regenerate, then when I zoom back out, I need to regenerate. When I move the picture to keep tracing, I need to regenerate.
This was not an issue when I was using 7.2. Is there something I need to "turn on" is this a glitch?
We don't use this feature much, but when we use it, it is taking twice as long to trace due regenerating every time I move the picture to trace or when zooming in or out.