Went to the instruction manual for this airframe and your CG is set at 4" back from the leading edge of the wing. This is just a starting point and they allow a + or -minus 1/2" CG range. The balance most be performed with the fuel tank empty. If all this fails to remedy the problem then purchase some stick on lead weights and attach them below the horizontal stabilizer. I find it hard to believe that you are having an issue with this basic trainer.