Calendar invites not appearing

Okay, so I'm experiencing a new, strange problem when trying to share calendar entries between two BlackBerry devices. A 9650 running OS 5.x and a 9650 running OS 6.x...

I put OS 6 on my mother's Bold and left OS 5 on mine (can't give up my theme yet!) and now it appears when I send her a Calendar invitation it never arrives on her device, nor do her Calendar invitations show up on my device. We are both using Gmail accounts and the invitations are accessible if we login to our respective Gmail accounts on our computers.

Can someone tell me that my only choice is to get both devices back on the same OS? Never in my wildest dreams would I expect the OS version to matter, but it's the only thing that I can think of that's causing these invitations to not show up on our phones.

Any ideas?