Drag and Drop conversion automates the following steps:
1. open the file in the default reader
2. print on the pdf printer
3. load the pdf in the UI
Sometimes it can happen, that pdf24 is unable to change your default printer to the PDF24 printer in order to convert the files. If this happens, the file is printed on your default printer.
Solution:
Change your default printer and use your PDF24 PDF printer as default. This normally helps here.