This happens in scenario where the user comes on the website for first time and leaves after seeing just 1 page , on the very first hit there is no value for visitorID as thats the time the visitorID is assigned.
Hope this helps.
When you say visitor ID report can you confirm if our referring to some variable itself where you are storing the visitor id or is it the one from DW report, as both will have its own reasons.
The visitorID (if you mean the ID from the "Experience Cloud ID Service") should be set before the server call for analytics is sent (because Analytics needs an ID for the user to match the hits in the backend).
I would do the following:
1) get on page and inspect the server calls if the evar is set in each call. Best case is to set "evarX=D=mid" to let analytics backend put the mid in the appropriate evar.
2) get a raw data feed and inspect the hits where the evar is not set, compare to "visid_id_low" and "visid_id_high" which store the visitor ID. this way you might find a pattern where the evar is not set (maybe on custom link instances, exit/download links, certain pages, certain users)...
Agree with ursboller,
Try to narrow down the issue. Try to sub-relate the pages(Single Page or Group of Pages or Entire Site).
I doubt the cookie domain support, if cookie is not supported, 'mcid' wont be generated making the 'eVar' null.
If there are no issues, share the domain, we can validate and let you know.