Why do some of my YouTube videos drop frames on a new IT network?

My IT department are upgrading Citrix (which is our internal IT environment, I think) which is a direct result of myself and other colleagues complaining that video playback was poor quality, not just on the internet but also playing directly in the environment from videos hosted locally. The IT guys called me down to show me some of the videos playing in the new environment on Youtube and they looked good, but the videos I have personally been creating and uploading to YouTube for the company are lagging immensely and the video is dropping a lot of frames but other HD videos appear to be playing fine. It's got me worried that I've been using the wrong codec/frame rate/settings for uploading video. However when I play videos in our old environment and on my home network they work fine!. So what is going on here?

For info my footage is shot on GH4, 1080p 50fps, mixed with stock footage of 1080p and 4K resolution varying from 23.94fps to 60fps, timeline is 50fps, exported as H.264. Yes I am aware I have no idea what I am doing as far as working with multiple fps clips in one timeline but the point is the video plays fine anywhere else but the new IT network!