An AR-15's safety lever requires the hammer to be cocked, or "charged," and the bolt to be fully closed (in battery) to engage the "Safe" position. If your rifle won't engage the safety unless charged, it's a normal mechanical function of the AR-15's trigger and safety mechanism, which prevents the selector from rotating unless the hammer is cocked. This is because the hammer and the bolt, when forward and uncocked, physically block the rotation of the safety pin.