iCal invites going to email and alerted with iCal

Hi,

Wondering if someone can offer some advice.

Ive been experiencing some issues with iCal around the invite function.

For example, my girlfriend often uses the invite function to share things with me on her calendar. Ive found that sometimes as soon as she creates the event and adds me as an invitee, I get an alert on my iPad and iPhone, MacBook and iMac and I can accept/decline etc. However, I sometimes get an email instead of the alert. Does anyone know why this would be? If this happens and I click on the .ics attachment on the email, it opens iCal and I can accept, however when my girlfriend looks, it doesnt look like I have actually accepted when I have.

I use iCloud and have an @me.com email address but also my @sky.com email address is setup with my apple account. At first we thought it only alerted me on the iCal app when she sent to my sky email address but this doesnt appear to be 100% everytime.

Im really confused by this now and would appreciate any advise or anyone else who has had similar problems.

Thanks in advance.