Well, my first thought would be that the last time the mag was disassembled for cleaning the spring got one too few turns to tension it. BUT...I've had a few 10/22 and 77/22 mags get weak-springed as well. It might just be full of crud though too...
You could try winding another 1/3-turn into the spring...but it's probably already fatigued and this will only stretch it's lifespan for a short while.
To disassemble a Ruger rotary mag, you need to remove the allen-head bolt that runs through the center of the mag (fore-and-aft). Once the bolt is removed from the mag, lift the endplate away from the mag body. Be careful as it should be under some tension. You'll also need to hold the steel feed lip block in place as you remove the endcap since the follower should be pressing against it.
The coil spring inside should be hooked to the endplate on one end and to the follower on the other.
Here's a page I found via a quick google search that outlines dis- & re-assembly pretty well.