Hi,
Please, upload some pictures so we can try to help you.
It is possible, from what have you described, to be from a dirty laser or from some debris inside the developer housing.
Gabi
This was taken by reducing 50% colour of Magenta
Hi,
Try to swap the drums between them to see if the problem move to another color.
If this did not solve your issue it will be better to have a tech to replace the magenta developer housing and magenta developer material.
It could be also from the laser unit.