What lens are you using? It is probably because your lens has a very narrow maximum aperture.
What lens? If your D3100 is using an f/5.6 (or f/5.8) aperture lens, that's as far as you can stop it down.
I don't know the D300, but similar Nikon DSLRs have a secondary control dial on the front, not far below the shutter button.
See link below-
which pairs of angles in the figure below are vertical angles
Most Nikon lens will work in complete manual mode, but to get the most from the lens (auto-focus, metering) stick to Nikkor lenses with AF or G in the title. Full comparability chart in the related links below.
70
84
6.8
true
This is the website, in the Related Link below, you have to go to if you want to figure out how much you will get in your unemployment check.
Check link below
The denominator.