I think, as I just said quickly in another post, you should check your arm angle in relation to your body and your stance. I'm not going to try to be an armchair coach, but simply state again here your body needs to be in a certain anatomical position to hit a flat shot hard enough to get the effect you are looking for. If for example, you leave your stance too open or have your elbow in too close, you aren't going to get the pace you want. Check the relationship of all moving parts to your stance the next time you try to hit one to see if you are in a bad position to produce flat power.
Still with the Head LM Radical MP/OS. The dying art of the 1HBH. Gamma X-2. Would a higher-end machine get me more forum "cred"? ;)