Not sure what exactly you mean by "darker"... but if you gotten it as deep and rich as you can following the normal instructions on the package, I would suggest one of two things.

1. Try using less water in your dye bath. That might increase the concentration of the dye and force more into the fabric.

2. Try adding another colour to the mix. Maybe Wine?

I have no idea how either of those would actually work. They probably would just work in my head So if you try my recs and they fail, don't blame me

Oh, and be sure to look at the material the socks are made of. Some synthetic materials don't take to aftermarket dyes very well. So if you've got a blend, you may only actually be dying certain parts of the thread and not others, which would make the finished product a wee bit lighter than if it were, say, 100% cotton or wool.

Maybe try adding salt if you haven't already? Salt helps the fabric take on the dye better. So if you haven't been using salt, you might not be getting the full effect.