I have been having this problem, too, more especially recently.

Yesterday, I was searching for LSE-SAN and clicked on every day in January that it said was available- nothing came up. Then I went to searching for MSP-SAN, and on dates that the calendar said, "Not Available"- there were 8 partner flights available.

However, it worked flawlessly for recent LSE-PHL flights. I suspect it gets hung up on partner flights and/or flight plans with more than one stop. For example, it never offered me any flights on the LSE-SAN routing that involved two stops; just "not available" showing only the NW one-stop flights.