Open up the zip file you created when exporting the templates out of NP 16. Double click on the folder "templates". What size (KB/MB) is the missing template you are not seeing in NP 17? If the missing template is larger than the others then you may have some items within the template that are not being used that is causing this issue.
When you open the Powerpoint template editor in NP 16 do you have anything listed under "Cells" on the left hand side of the template editor screen? If not then my solution may not help. I had this same issue where individual fields listed under "Cells" in NP 16 that were not being used in the generation of the report caused the template to not be visible in NP 17. Once I removed these fields and re-saved the template, then re-exported the nsq the template was visible when trying to re-add it in NP17.