From my experience with this problem, it is caused by having 2 monitors operating on one computer- though I have not found a way to directly fix this problem. Instead, I have found some workarounds for it that work, in my opinion, just as well.
1. Alternate Snipping Tool selections- If you press the drop-down arrow beside the "New" button in the snipping tool window, you will be presented with a few options for how you want to capture a screenshot. "Rectangular Snip" is not working for you right now, so thats not an option, but fear not- you can still use "Free-form Snip" mode, which is essentially like rectangular snip but... not restricted to a rectangular selection. If you are selecting a window, then you can use "Window Snip" mode and select the window that you want to capture, and all of the windows above it, too, if it isn't frontmost. Finally, if you want to capture your WHOLE DESKTOP, use the "FullScreen" snip mode. This will capture all of the content on BOTH of your monitors, and then if you wish to do so, you can size down this screenshot in a photo editing software such as Adobe Photoshop or Paint.
2. PrintScreen- The "PrintScreen" key (Located above the "Insert" key on your keyboard) simply does the same thing as the "Full-Screen Snip" mode in the snipping tool does- it captures both monitors, and everything on both monitors. The only difference here is that this does not need any applications to be open initially; instead, the captured screenshot gets copied to your clipboard, and can be pasted into any Email, Chat forum, or even a SuperUser question. This screenshot can be placed into, again, a photo editor for editing or cropping.