Well good news is you just found your problem. 20-25 stack of voidform is semi-decent for your standard voidforms but once and a while you should be able to get a, for lack of a better word, "super-voidform? lol" where you can get it up into the 30 and even to 40 stack range. This all comes from dispersion of course. Use dispersion shortly after going into voidform to slow down your insanity rate which then lets you stay in voidform longer and get more stacks. On top of this it's one thing to say you're using voidbolt on cd but technically you really need to using LITERALLY on cd. The difference being even delaying casting a spell for half a GCD if voidbolt will come up in that time. It's faaaar better to do nothing for half a GCD waiting for voidbolt than to cast mindflay and be half a GCD late on voidbolt. Sounds counter productive but voidbolt really is that important.
Hope this helped.
EDIT: Just looked at your armory and someone correct me if I'm wrong but I'm pretty sure Void Ray > Reaper of Souls UNLESS you're taking StM