Messages going astray

Some messages posted to our mailing list aren't reaching everyone. I
discovered this when a message I posted didn't then turn up in my inbox. I
also keep a Yahoo account subscribed to the list and have found another two
messages other people had posted that did not reach me.

I have no idea how many other messages might have been going astray -
presumably others in the list may not be receiving all the messages but
won't know it.

What might be happening and how do I sort it out?

I contacted the web hosting service who say they don't deal with scripting
problems.

Thanks

Clare