Cleaning with WD40 sometimes won't give you good result. Do this following method.
1. Removed the silk screen and soaked it in kerosene and dry it with cloth, check any foreign materials between the two screen.
2. Since silk screen is removed clean also the drum cylinder the metal parts with cloth and kerosene.
3.Check the pressure roller, this roller is located under the drum. This roller press the paper into the drum. Uneven roller cause the image not even specially the middle part of the copy.
4. Lastly, if all the above solution fails you need to dismantle all the silk screen ang the metal screen ang clean the roller inside the drum and adjust the thickness of ink by adjusting the squeegee roller distance between the doctor roller. Used metal filer gauge the distance should be 0.3 - 0.5mm.

Remove the tetron/silk screen and clean it check for foreign particles on screen and drum

Soak towel in diesel and wrap it round drum and leave overnight works for me