Comments not appearing for some users

Hello,

I am experiencing a weird issue.
After someone leaves a comment under one of my posts everything seems ok for the user but the comment never appears in WP dashboard as pending approval.
The crazy thing is that it works for some users – for example when My host provider (based in Canada) leaves a test comment it appears in WP dashboard as pending approval and I can see it.
But for many other users (for instance across Europe) it doesn’t work – if they leave a comment It never appears in the WP dashboard.
I really don’t know what to do.
I am using Shout WordPress Theme.

The page I need help with: [log in to see the link]