Right, here's what to do.
1. On the launch screen, click 'Manage Content'.
2 .Then, at the top in the bar where it says 'Installed', click on that, and select 'Custom...'
3. Click where it says 'Asset KUID' and select 'Missing Dependancies' from the dropdown menu.
4. From the box where it says 'False', select 'True'.
5. Click 'Search'.
This is now showing all the content that is missing dependancies- ie, it doesn't have all the parts it needs in order to work.)
If you right-click on one of the locos with the words 'Missing dependancies' next to them, and click 'View Dependancies' from the menu that comes up, you'll be shown a list of the dependancies for that piece of content. Most should say 'Installed' next to them. Some will probably be in grey and say 'Available for Download', in which case, right-click on them and hit 'Download.'
If, however, some are showing up as 'faulty' or all the dependancies are showing as being installed, go back to the screen where you have the list of content which is missing dependancies (ie the screen that came up at step 5). Right click on the loco again and this time select 'view errors and warnings'. It'll probably then say 'KUID:0:0000:0000:0 is unknown', but those 0000s will be replaced with the KUID number of the dependancy in question. In that case, come back to the forum with the number of the KUID that's missing and post it in a new post in General Trainz and someone will tell you what the missing KUID is and how to find it.
Hope that helps,
Regards,
Alessio