So, I’m trying to create a simple movement trail for a third person character. However, while the anim trail DOES show up, it shows up without a material (it shows as the basic grey checker pattern) and I cannot figure out why. Also, it only shows up if i view it from one side. After turning on two sided in the material I am using, nothing changed, so thats why I think that it is not even using that. I have the material assigned in all the necessary places: in the Required part of the particle emitter and in the particle component of the third person blueprint. I can only imagine this is a bug, as I cannot see anything more that I could possibly need to do to fix this. If someone knows something more I can do, please let me know.
Here is an image to help show that I think everything I am doing is right. If someone sees what I am doing wrong, please please let me know.
I recently had the same problem as yours with seemingly nothing wrong, This was because nothing was wrong.
I fixed the problem by opening the animation that the Anim Trail was attached to and manually moving the animated object. After returning it to its original position the trail began to work properly and loaded the correct material. I’m not entirely sure why this fixed the problem, my guess is it has something to do with properly loading the Anim Trail. Regardless it certainly resolved my issue. Hopefully it does the same for you. Good luck 
I’ve recently run into this problem myself. However, like the original post, I’m activating the animtrail through blueprint, and don’t have it connected to any animation that I could manually move.
Does anyone else have any insight on this?
You need to check in the materials you use for your trails : Usage-> Used with Beam Trails
Enjoy !
Thank you!