Aspect ratio

I use Movie Maker in Windows 10, and I have noticed a problem.  I use Format Factory to convert old DVDs, made with my camcorder, to digital form, and I want to use Movie Maker to edit them.  Format Factory divides the titles into five parts and assigns them to the FFOutput folder, but I noticed yesterday that part 1, for example, shows the aspect ratio as it should appear, but when I add part 2, which is simply a continuation of part 1, the aspect ratio changes.  The widescreen format suddenly changes in part 2 to a different ratio.  The left and right sides of the picture are now flanked by wider black bands than found in part 1.  I cannot tell that any part of the frame is missing, but I don't understand why the aspect ratio changed, especially when part 1 of the scene is as it should be, but its continuation in part 2 appears to be cropped on the right and left sides.  Can anybody tell me how to keep the widescreen aspect ratio?  Thank you for any help.