Looking to fly LGA --> DAB
Would like UA into CLT. CLT --> DAB would/should be AA
I would expect LGA --> IAD --> CLT --> DAB

No matter what combination I do I cannot get this linking of routes to show up. It will only return via ATL to DAB. I want to see CTL & ATL --> DAB for price comparison in the results.
If I exclude ATL (-CITIES ATL), it returns no results.
I know this route is possible because I can do two separate searches on Google flights, but I would prefer ITA so it returns in one search.

departure routing codes: UA+ F?
return routing codes: F? UA+

What am I doing wrong?