I purchased a 52" Polished Brass Lakeside II Ceiling Fan along with the extended 24" downrod specified to go with this fan. When my electrician tried to attach it to the plate he found that the holes did NOT match up, and there was no way to afix the downrod.
He had to spend extra time improvising and making new holes to make it work. Why doesn't the downrod fit the fan mechanism? It's a Harbor Breeze downrod made for this model fan.

He will need to use back plate that came with fan. Fans usually aren't that difficult.
Try fitting on the ground. Usually fans have provisions for various screw distances.
Maybe wrong part came with fan?

The longer length downrods are sometimes designed as generic to fit any model fan they sell, or existing fans you might have. Usually they can be modified to work with pretty much any fan if the ends are theaded and they are the correct diameter.