I've got a radiator that will get a little warm, but not hot like the
others. What might be the problem? I see there's some sort of valve
on the side, but I don't have the right tool to do anything with it.
I can probably borrow some needle-nose pliers if turning the valve is
the thing to fix it. Suggestions? Thanks.

If its at the top and you have hot water, not steam its a bleeder
valve, open it till water comes out while your system is running