Gmail not downloading pop mail or mail disappearing

I have setup a couple of gmail accounts for clients as a central repository for various email accounts. Clients have complained they have not received some emails or they have been substantially delayed.

Looking at the gmail check mail settings for the pop accounts, sometimes it says mail has been checked 53 minutes ago, or some other long period. I have not found any setting or information (yet) on how frequently gmail does the POP account checks, or if you are meant to manually press the check mail now button (dud)

In any case it is far faster and more reliable to set the other email accounts to forward email to the gmail address, that way mail comes through instantly without having to wait for the pop downloader to get them.