When using a POP email account with multiple devices, email messages may only appear on the first device that receives them. This occurs when an email client is configured to automatically remove email from the server once an email client has received it. You can configure your email client (or clients) either to keep a copy of the email on the server, remove it after a certain number of days have passed, or keep it until you move it from your Inbox.
Apple has a guide on setting up various devices to work with pop here: http://support.apple.com/kb/HT3228#
