MFC 9320/9330 vertical lines with different density and bad quality in general

[blue-brother]

Hi all,

I've been trying for several days to follow the recommendations how to fix my printers in the brother webpage, without success.

I have 2 printers in my office. The first one is a MFC-9320, and a couple of years ago, I bought a MFC-9330 also.

I'm having the same printing issue now in both. It started in 9320, as it's older, and then we sent all the colour jobs to the 9330. But now this one is having the same issue.

When something in colour is printed, I can notice some vertical lines with different colour density. Also, the definition/quality is not the best.
I have tried changing the toners, and nothing. Then I tried changing the drums some days ago, but nothing has changed.
I followed all the recommendations/fixes in the brother website, but still having the same issue.

I'm uploading some pictures so you can see my problem.
It's the same in both models mentioned.

Can somebody help?

Thanks in advance.