what application are you exporting from ?
What steps do you follow in Word (that is, what do you click, as I don't see an Export button in Word)? Which version of Word? Which version of Acrobat Pro or Acrobat Standard? (Not Acrobat Reader)?
I'll echo the same questions as "Test Screen Name" above:
1. Are you using the Acrobat toolbar in the Word ribbon to create the PDF?
2. What version of Word?
3. What version of Acrobat?
4. What Operating system and version?
If you're not using Acrobat, we may not be able to assist here, but perhaps we can steer you in the right direction. Please clarify your process, and clarify all the relevant software information as noted above.
If these graphics are in the header or footer of your documents, I've seen instances where removing the header/footer and re-creating it in Word solved the problem of a missing or corrupt graphic element.