Can you describe in detail exactly how you are approaching this? How do you "try to find the original BPM" ?
FWIW: Here I may approach this by first of all creating a new project, with tempo set to "Adapt Project Tempo". I then import the original file, either through Browser-Audio File-Add Audio File, or simply drag and drop the file from the finder. Logic then proceeds to analyse the file, then asks if it should show the smart tempo editor. Whether the results actually make sense depend on a number of factors related to the piece of music. Some very legato piece with long slow strings and no clear rythm may not result in a generated tempo map that makes little or no sense, often the tempo map will be broadly correct but may need to be adjusted in places. In that case, you will have to manually adjust the tempo line showing in the tempo track above the main area. Sometimes Logic has found the correct tempo, but beat one on the song ias actually interpreted by logic as for example beat 3. In this case, simplky drag the audio region 2 quarter notes until it "sits" correctly. You can add "nodes" simply by clicking on the blue line, or in more difficult cases, resort to beat mapping. Oh, you may also have to adjust the time signature. Logic may not immediately recognise this. Again, up in the global tracks area, right click, add "signature and choose whatever you need. This is also vital iin songs with varying time signatures.