Some elements not visible in sheet views

I generated plan sheets via named boundaries - but some of the elements from my attached reference files aren't being displayed in the sheet views - they're invisible. In other words, I can hover my cursor over and select the elements to view their properties, but I can't actually see them.

FWIW the problem isn't isolated to just one or two of my references; each reference has at least a few elements that aren't visible in the sheet.

Please advise.

Thanks