Another printing fault is light pollution known as fog. This is assuming it hasn't happened in the camera.
|The dark line you see running up this picture|
it the result of light
getting to the light sensitive material. Fogging .
● Your safe light is too close to the enlarger and or the wrong colour.
● It can also happen if your enlarger leaks light when being used.
● Badly stored and out of date paper can also be fogged.
The causes of these faults, also point to ways of curing them. This post suggests a way of detecting light induced fog.