Mixdown query

Hi To all Cubasers
I am getting into mixing as a beginner and have a question

I got my mix to a great level and quality over 29 tracks using izotope plugins.
I have mixed the song and got it down to one track.
Now i want to have a go mastering it.

After i mixdown at about 17 Lufs the result is much lower at about 30 Lufs.
I would have thought that after mixing down your tracks it would be the same volume ready for mastering
I do use a fresh cubase 10 instance. Should i put that wave track back in the same project and mute the others
I do use Sonarworks and disengage that when mixing down.
I just need a little guidance here as i am confused why the volume change after mixdown.
It means that my Focusrite has to be turned up to get a decent volume and my reference track then blasts me to kingdom come
I have to rebalance all the volumes again.
I am not looking for loudness at this stage as i can use a limiter at the mastering stage.
I think that is right, but. I must be doing something wrong.
Any help appreciated

All the best

Are your masterbus set at 0db volume or is it turned down?

Yes the Master bus is set to zero or unity.
I did gain stage all tracks in Cubase and went from there with Izope Neutron etc.
I have just only got the Neutron /Ozone package, so there may be a few thing i have to learnin there.
I went through the motions ofletting Neutron here the track and also got around the tonal plugin and all loked well balanced until i mixed down and then i messed up and got in a pickle . I having a bash at covering Mr Blue Sky. A devil trying to get the choirs just right. Otherwise not too hard playing it.
It is the flaming mixing that takes an age!! well at the moment. lol.
I am starting again as i hated the kick drum nuances. Not right for the song i am doing on Keyboard Genos
Now i have learnt it is worth splitting a style drum track into separate parts.
I am wondering whether the cubase gain staging and the izotope relay is conflicting when adjusting both.
Should i just gainstage with relay or Cubase?

All the Best

All the best