he drum is not being cleaned properly between prints. Depending on your printer, that could be the fault of the cleaning blade, the cleaning drum, or possibly an erase lamp, if your printer is very old.
Again, depending on your printer, this could be corrected by changing the toner cartridge (not refilling it).
You may also have toner spilled within the printer - have you tried opening it and cleaning it manually?
Toner leakage, it happen to me when I refilled, but I have only 1 tiny spot, not a big problem for me