A regular 9-volt should be good enough for you Icon-E, as long as there duracell or energizers.
It could be several things that are causing your problems, the sear spring might be too stiff, the solenoid might be replacing. Some Icon's came with a sear spring which made the solenoid overwork and that can make the solenoid weak.
Also check that the solenoid is fully forward in the frame, sometimes they can come lose, use a flat head screwdriver and push the solenoid fully forward and retighten the set screws on top.
I would either send your marker to Paintballsolutions or call them for a new soldenoid and sear spring if your problems continue.