QuickLook does not preview .txt files
You're using the QuickLook Plugin with whatever app you've set as default to open text file. You can probably find that in Your_Application.app/Contents/Library/QuickLook.

As @Julien said:


  Generators embedded in applications are always preferred to plug-ins in ~/Library/QuickLook (which are preferred to the ones in /Library/QuickLook, which are preferred to the ones in /System/Library/QuickLook).


And there is no way to change that preference order. If you want to make sure it's the custom generator you can see the file with the default QL plugin with the following command in Terminal:

qlmanage -g /System/Library/QuickLook/Text.qlgenerator -c public.plain-text -p &lt;path_to_text_file&gt;


You can try to delete the app's QL plug-in, that could work but GateKeeper may stop you from opening the app, since it won't match it code signature.