+1 on searching through Google Flights first, then either trying to replicate on DL.com or using the click-through. I suspect the DL search engine is programmed to give preference to DL metal for whatever reason, and it absolutely does not show all options that DL is able to ticket.

This restriction doesn't happen so much on simpler US-domestic tickets, where DL metal is really the only option. My guess is that to save processing effort for the website, DL.com truncates the results in some way when someone is searching for int'l itineraries that may have a very large number of options and partner possibilities. They've unfortunately chosen to invest instead in a gaudy graphical interface that is more form than function.