pixelated slides when screen sharing

Hello, 

i have an issue when i screen share powerpoint presentations. the first slide I show is fine, but after clicking on, the slides appear to get stuck and become patchy and pixelated. 

i have tried:

uninstalling and installing the zoom app

restarting my laptop

restarting my internet connection

sharing with the 'optimal view' clicked and unclicked in full screen and non-full screen mode.

happens when I am in my own meeting and when I am in others meetings.

nothing seems to help! any suggestions please? i have a Macbook Pro.

thank you in advance