I'm going to go with CG too. When I ask about CG location I am looking for one of two answers. The answer I hope to hear as it applies to this case would be: I set the elevator throw to 3/8 travel and the airplane wouldn't turn tight enough and was hard to slow down for landing (nose heavy) or even with 1/4" elevator travel the airplane needed lots of pitch corrections between pylons but landed really nice ( tail heavy ) . Giving a hard measurement as per plans/instructions means that the CG setting was not a result of flight testing. As Scorion has suggested you may want to shift the CG forward before doing any more shimming or cutting. Even on my aerobatic models that are built with everything strait if The model requires elevator trim I will look to CG first.