Hi Don
Normally when you right click on a route and tell it to download it will pull all or most of the dependencies in with it. There may be a few missing if they are third party assets from an external site and sometimes the DLS fails to find all of the dependencies for a few assets. A similar situation applies to rolling stock assets.
With regard to the missing assets for the route, have you tried right clicking on the installed route in content manager and selecting download? This should download all missing assets available on the DLS. Alternatively you can right click on the route and select "List Dependencies" which will open a new list box showing all dependencies. Click on the column heading "Status" and it should sort them based on their status. You can then select the whole block of assets available for download, right click and select download.
I have quite a few routes with missing dependencies which I'm unable to find. Unless it's something major such as track, signalling or an industry then I have a look at the route and see if the missing item is obviously missing. If it's something like one tree out of many then I don't bust a gut trying to find it, I just ignore it.
Regards
Brian