You may need to install the samsung usb drivers. There is a difference between connecting your tablet to your computer and windows reading it as an MTP device, and connecting it in download mode which the computer sees differently and will need drivers for it. You can find them on the Samsung support site in the download section.
Phones & Tablets: Galaxy S4, Galaxy Note 8", Evo 4g LTE, Evo 3D, Acer Iconia A500, Samsung Galaxy Tab 2 10.1, Ipad 2, Nabi 2,
Rom: Rooted Stock Odexed, Hotspot Mod, Xposed, Wanum