Live Mode Auto Focus Problem with 50D

I have been using AF with Live Mode on my 50D with a Canon 100mm Macro with no problem. Then I put a 24-70 L on it and AF stopped working with just using the partially depressed shutter button. I can get it to auto focus by depressing the AF button on the back of the body but has anyone experienced a problem getting the 50D to auto focus using just the shutter button?

Thanks

This is normal.
Live Mode is a contrast detect mode. It uses the contrast of the image to lock in focus.
You will see this is noticeably slower to focus. If you want to still use the half-pressed shutter button to focus while using live view, then you need to switch live view into the Quick View mode. This will flip the mirror down, turn the screen off and lock the focus, then the LCD screen will come back on. The only thing is, you have to make sure you select the correct thing to focus, otherwise you'll be going back and forth.
For me, if I'm using live view, that means I have time to take the picture. So I just use the Live Mode option, where it keeps the image on the screen while it focuses.

Thanks for the information, I appreciate it.