an interesting test to do... when you have rendered the image. lets say as a .tiff. open the .tiff in photoshop imediatly go to the menu " image" and "choose image size" does it say it's 300 dpi. if the answer is no, it will print at the resolution displayed. don't resample the image just change it to the correct dpi in PS and the output size will change. also check your page set up, the user imput here may be effecting the printer's output.