Make sure you can actually place the consist. There can be multiple causes that prevent you from doing so.
1) You are placing the consist where there's not enough room.
This is caused by a short siding, junction set wrong so that the consist can't fit on the tracks, and also because there's another consist in the way.
2) The track is on a different layer than the one where you're trying to place the consist.
3) The track somehow became corrupted.
There's a bug that causes the track to change kind from track to something else. The cause is unknown but appears to be related to the database. When this occurs, you can't place track objects, attach other track to the already placed track, and you can't place any consists.
The fix for this is to open up all your third-party track for editing in Content Manager then submitting it (not reverting) to force the database to update.
You can test for this by creating a test route. We recommend you do this anyway to check textures, experiment with techniques, and to test assets.
Create a simple loop with the track you are using on your route and place your consist.
Does the consist place down fine?
If it doesn't then you, then you have the track issue. If it does, you have something wrong with your route, and that's where you start checking layers, and track length.