Video Export Problem

Hello, I'm new here so hope someone can help me. I have created a video using 17Beta and it exports fine to my desktop and plays as I expect but if I try to share it and send it to my phone or iPad when I try to play it I get a series of still images with blank parts to the video, no movement and the sound in the back ground. Its like it is only taking 5 or six stills from the video and everything else isn't showing up and yet it works fine on my desktop?

Apologies if this is a stupid question but I'm fairly new to Resolve and have never had this problem before.

Thanks in anticipation of any help.