I have long assumed that when you search "CORRESP" by issuer name, you would see all correspondence filed on EDGAR with respect to that issuer.  I was therefore surprised when I recently searched for a letter responding to the staff's comments but couldn't find it either in the complete list of EDGAR filings or by searching "CORRESP" under the issuer's name.  The SEC staff was very helpful in explaining why this occurs.

If a response is filed as a separate filing, it will appear as "CORRESP" and will be listed when you search that issuer's EDGAR filings.  If, however, the response letter is filed as part of another filing (e.g., an amendment to a registration statement), then it won't appear when that issuer's EDGAR filings are searched.  After doing some follow-up digging, I discovered that the SEC has in fact disclosed this anomaly on its website:

Note, however, that filers often send a reply to staff as part of another filing. Such responses cannot be searched directly with the current EDGAR interface, though they will be listed on some search results pages, designated by the term "[Cover]" in the document description.

Nonetheless, I wonder how many others make the same erroneous assumption that EDGAR's list of filings for an issuer includes all response letters?

This also presents an opportunity to work the system.  Although filing a response letter as part of another filing means that it can be found on EDGAR right away, it also means that it will be somewhat "buried".  Some may therefore be willing to trade immediate availability to gain reduced availability.