SD Initialization Failing
I also use a Seeed Studio SD Shield, and used to have the same problem. To rectify this I have to insert the SD card into my PC and Format it. This almost always helped. If you have a card that has too large of storage, the Arduino will take too much time to check the full memory for information stored on it (initialize). Some SD card readers require a card of 2 gigabytes or less to work. Check the Data Sheet for this limit (if there is one). That should do it for you.