Frank Skomial , Feb 09, 2012; 03:56 a.m.
Most likely it is:
1. your shooting technique.
2. your lens.
Even with a 300mm Nikkor one can get soft pictures with poor lighting and poor shooting technique.
Inspect your pictures EXIF data and see if the shutter speed was slower than 1/450 sec, so you have the likely reason.
Shoot at shutter 1/500 to 1/1000 sec and hold lens steady.
Actually test your lens on a tripod, to see if it is the lens, or your technique?
most likely combination of both reasons..?