iCal won't sync with iPhone 4

I am having trouble syncing new events onto my iPhone. I have made some new entries on iCal on my iMac but when I synch my iPhone 4 these new events are not on my iPhone calendar. I have tried ticking the box to overwrite the information but even after I do this it still won't put on the new events, but oddly the other events are still on my iPhone, some being old entries that I have had set up for ages (reoccurring events) and some being entries that I made on my iPhone.

Why won't these new events that I make on my iMac synch with my iPhone?