You're probably not hitting it out in front enough. With a straight arm forehand you hit considerably further out in front.
If you catch it late, your elbow is going to take a lot of the energy from the impact because you don't have any leverage.
You need to keep in mind that a straight arm forehand does not mean that you hit with a completely straight arm all the time. Think of it of a different ideal contact point. If you mess up the timing slightly, always adjust the bend in your arm to make it more comfortable. Even Fed does this at times, so should you.
Last edited by TheCheese : 10-28-2012 at 02:37 PM.