Cant receive Open Stats some domain

I'm sending some test campaign to our test clients. But cannot receive open stats for some domain., send open stats but some other domain can't.

Is there any little trick for this?

Thanks in advance,
Do you receive those emails in inbox for gmail/live ?
Do you allow loading images from remote sources in those clients? is marked my email as junk, when i open mial in junk folder, open stats counting. mail is in gmail inbox and no problem.
All mail reads by Mac or ms outlook 2013. But some other domain (e.g. cant send open stats. There is no any restriction to download external images all email clients.
Well if it works on one client, then def. isn't a problem with the app.
if the end client mangles the emails and removes or alters the tracking pixel then there's nothing i can do.
Problem solved in mail server side. Some mail server with spam controller is (like mailscanner) blocking tracking pixels (like 1x1spacer, space.png etc) and replacing with their links. After the disable this option in MailScanner, problem solved.