Hi Jamie,
Can you open that PPTX, copy all slides (CTRL A and CTRL C), and paste all the slides in a blank new power Point Presentation (copy with keep source formatting), then save that file as PPTX and import that in a Captivate Project of the resolution that you use.
Also try with importing PPT instead of PPTX of that presentation in Captivate, if you are not using any transition or animation.
Thanks.
