Web Pages Losing Page Rank

I'm still out checking some of these low producing pages. I've added comments to some of the previous posting, so you may need to click the title to see the added comment.

Any way I'm finding pages that were added to the site a year ago that are not showing a page rank. I know it could be a google issue, but I don't want to take that chance. A few examples of pages that appear to have lost their page rank [all from the How to Design an Equipment Chassis]:

Environmental Alarm Manufacturers,
Temperature Alarm Manufacturers,
Chassis/Card Slot Keying,

Why would the page rank revert back to a 'gray bar', err zero page rank?

Three Apple NuBus pages are also getting low page views, of course the bus has been obsolete for 10 years:
Connector Row 'A' pin out for NuBus,
Connector Row 'B' pin out for NuBus,
Connector Row 'C' pin out for NuBus,

Also XDR2 Memory Modules....

So once you get a page rank, you could lose it, but why?
The graphic shows that only 10 pages out of the 42 pages in the How to Design an Equipment Chassis get most of the traffic.