It's never too late to try.
A couple of trips we have made ours as we're driving to WDW. The only thing we've had trouble getting is an in-park character breakfast. We do try to be flexible, though. For example, we usually have several meals in Epcot, so I'll tell the cm the days we are planning to be there and which restaurants we want instead of planning for one specific time. (I hope that makes sense. I mean I'll as for Teppanyaki on Tuesday OR Thursday around 1 p.m.)