Problems playing videos from Macbook to Apple TV

I am having a frustrating minor problem playing my video files. Here is a summary, I wonder if anyone else is having the same problem:

1. I had a old 2008 macbook pro. Apple tv into the back of a TV, working on wifi. My videos are in itunes/home videos. They play over my wifi on a linskys N2500 dual band router. Everything works fine. I sell my old macbook pro to by a Macbook retina 2015.

2. Macbook retina airplays, no problem. It won't play videos. They load, then never start. Or just drop off trying to start and return to menu. I tried a song from imported cd's, and that plays jerkily, stop start. I thought, FECK, the Macbook processor is too weak. I erased and installed the apple tv. Same problem.

3. I called apple support. They suggested erase and installing, incase a software update was glitchy. I have not migrated anything onto this new computer. Just iCloud sync.

4. Still not working. I went to Genius bar. They set up the mac and the apple tv, and it worked! So, the theory was that somehow my settings on the apple tv. They wiped DMU (with network settings erased) the apple tv, and thus my router's settings would be new.

5. Back home. Same problem.

6. Back to genius bar with Macbook retina, apple tv and my router. We ran all sorts of tests and sure enough, my files will not play when using my router. This seems weird to me. I can stream via mirroring. I can play a video in quicktime player via mirroring (almost full screen). Then when playing from iTunes no video or music file plays.

To deifinitevely play out the tests, we transferred a video file that plays fine to a macbook pro used by the store, we joined my router plugged into the stores ethernet: thus Macbook pro - apple tv over wifi on the N2500 router, and same problem!

Why would iTunes do this, and why did it work on apple stores wifi and not on a very good spec'ed router. Why did it work on the macbook pro a week ago, and now, the only new ingredient is the Macbook retina.

Stubbornly confused as to what is going on. Has there been a software update in the last week for either apple tv or OS?

Thanks

Ross.