I've done it quite successfully with plants like stevia, or turmeric or ginger etc. The solution is simple - just keep the blender on for longer! Stop it from time to time and give the contents a shake-up, then keep going. It can take a surprisingly long time to get a real powder.

We have an herb grinder that reminds me a lot of a nut grinder-- a hopper that feeds into rotating "teeth" that grind the herbs not to a powder but to small flakes comparable to what you buy in those expensive cans and small jars.

Do you use a general food blender?

I use a coffee grinder ($25) and after grinding put through a sieve and get superfine powder.

Here is a link that might be useful: Grinding

I've used a coffee grinder also for herb/spice powders and blends, such as those used in Indian cooking. Many seeds, such as cumin and coriander, are especially tasty if roasted first.

Don't use one that's been used for coffee. It's nearly impossible to get the coffee taste out of it.