iPhone issue receiving changes from IMac/iPad2?

We have IMac, iPad2 and 2 iPhone4. We are using iCloud for everything EXCEPT mail.

When changes are made on the IMac/iPad2 in iCalender for example, they do not show up on the phone. If changes are made on the phone, they show up on both the IMac and iPad. They are set up the same, is there any explanation for the iPhone to send changes but not accept them?

Thanks,

Don