Entries Input to iPhone5 Calendar have stopped synching with iMac/iPad

Up until a few days ago, any calendar entry that I input to my iPhone5 would automatically synch to my iMac and iPad. It just worked without me having done any changes to either system.

Now, when I put an entry into my iPhone Calendar, it no longer appears on my iMac but it does appear on my iPad(Original Model).

As I run a small photography studio on my own, it is important that everything synchs - I'd rather not have to go back to using a Filofax.

Hope someone can help

Steve