"Can you explain how you are capturing the image of x-plane in the case where it fails with screenshot captor but works with the other tool?"
Both scenarios are:
I start X-Plane, which runs full screen and not in a window. I then proceed to fly a selected aircraft in a locale of my choice. i.e. KSEA Seattle.
When I move out side the aircraft to visually examine the aircraft in flight, I select a view point and then click Shift Print Scrn to initiates a screen capture. This pauses the simulator momentarily.
With Screenshot Captor, the select a rectangular area is initiated, but when the rectangle is outlined, the area goes blank, and becomes an empty black area which is what is saved.
With Gadwin Screen capture, Shift Print Scrn initiates the rectangular area selection process, and one selects the starting point, and then moves the mouse to select the end point of the rectangle. Clicking the mouse LB then anchors the end point. One can then move or re-size the rectangle. Clicking Enter then captures the selected rectangle, and depending on preselects causes the screen capture to be saved. Control is then returned to the sim.
I hope this helps.