If you're using a radio with dual rates, check the position of the dual rate switch. Check your firewall again, it's possible it's loose and you haven't caught it yet, I have superstar RTF and after a very light hit to the nose, the firewall came completely loose, it's not glued in very well. Does the plane perform well after it gets airborne? Is the grass cut short enough at your flying field? Grass that's too long will slow engine rpm and slow down takeoff roll speeds, reduced speed would make your elevator inputs less responsive. If the plane performs fine after takeoff, then you may just trying to rotate the airplane too soon. That's if for me hope it helps.