SDS Solution Preparation

For some reason, something that never happened to me before, I am having problem with preparing an SDS solution. I require 20% SDS solution to prepare an SDS-PAGE gel but when dissolving 40 g in 200 mL, this seems impossible to prepare because it doesn't seem to dissolve...looks like the solution is saturated with white crystalline 'precipitate' at the bottom. I stirred the solution for over 3 hours to no avail. I noticed that the 10% SDS flask which I had used for a couple of months now also has crystalline particles at the bottom but I would imagine that its because its been in the fridge for a couple of months now and precipitated. Cannot seem to figure out how to prepare 20% solution :/