You guys are correct about the throttle, I could only activate WEP at full. But I did notice it didn't matter where my RPM lever was, as long as throttle was full and I hit WEP it bumped RPM's to full as well. Moving the RPM lever had no affect for me while in WEP, RPMs stayed maxed out. I tested on a Spit XVI, I assume they would all be the same.
Going back to the OP if you were able to get into WEP then I don't think the issue is your throttle as you would have had to be at full throttle and your RPM setting wouldn't have mattered.