What kind of fittings do you have in your routing preference? If you don't have an elbow assigned that will work, it will not give an auto solution. Auto solutions are based on your routing preferences. Manually placing pieces of duct and comparing it to not being able to auto route isnt really fair.
I havent used the out of the box duct fittings for a couple years so I am not sure what is available or not available but make sure that the radius of the fitting is not restricting what you are able to place. Maybe the autoroute is trying to place a transition and it can't make it work which is why it isnt giving any solutions. Have you tried selecting the higher duct first and then the lower duct? If that doesnt work, select the lower duct first and then the higher elevation. Sometimes that helps. Pull your duct back to give more room between the open ends. Maybe it just needs a couple more inches to calculate a solution. If that doesnt work, trying increasing the distance in height between the two ducts and see if it will auto route then. If you can't get it to work at all then I would take a guess and say that there is something in your routing preference that is not allowing it to autoconnect.
Sorry I couldnt be of greater help, but I use the Eastcoast CAD addin and they have a auto transition utility that will generally connect any to ducts together. It will even put in elbows if the two ducts are at the same elevation but have different normal vectors.
