Calendar invites are not working for gmail or yahoo but are for Outlook. Why?

iPhone 5

updated OS

If someone sends me a calendar invite to my outlook... I get the invite in my calenar on my iphone and I'm able to accept or decline it.

However, if someone sends me one to my yahoo or gmail calendar, I only get an email notification in yahoo or gmail. The invite doesn't pop up in my iphone calendar.

However, once I accept the invite, that appt does sync to my iphone calendar.

Why?

Can it be fixed?

Oh ya... and 2 months ago... this was no problem. All invites were popping up in my iphone calendar. It just suddenly stopped one day.