Typically LibreOffice will be open MS Office files by just opening them, but that 'typically' is conditional on a lot of things. I agree on trying what JohC_21 suggests though, using the File menu option and see how that works out. As to why your files won't open using the drag-and-drop method you were trying, is LibreOffice in your Windows file type listing for docx files? If not that could be a possible reason why that's not working.
-- Also, do you use a lot of macros? MS Office uses VBA (Visual Basic for Apps) for macros while LibreOffice relies on Basic so if you have a lot of saved MS Office docs with macros that could be part of the problem. And/or if you embed a lot of MS Office docs into each other (i.e. a xlsx spreadsheet embedded into a docx file), that often results in problems too (including older versions of MS Office and with other third-party Office suites, not just LibreOffice).
-- Another thing to check in LibreOffice's Options menu is under the 'Advanced' sub-menu. Do you have Java (Java Runtime Environment or JRE) installed on your PC? (... and before some freak out, I'm referring to Java the scripting language support, not the cursed, and now basically defunct, Java Browser plugin)
-- and while you're in the Options menu, there are some MS Office specific options in the Load/Save menu that you should look into.