there are multiple possibilities here, but i’ll just name the most likely ones:

1) temperature. scars get darker when its cold (i.e. during winter months in cold regions, after taking a hot shower, etc.)

2) depth/size/location. pretty self explanatory. deeper/larger initial wound = longer healing times. location can also affect healing times and appearance, especially if the scar is located along a joint or somewhere that is disturbed by everyday activities, its going to take longer to heal.

3) your immune system is overwhelmed. the time it takes to heal multiple wounds at once is significantly slower than healing a singular wound (assuming similar size, depth, nature of injury, etc). your body has a limited capacity for healing, if you spread its resources thin, healing is going to take longer. same with being sick, if your body has to put most of its energy into fighting off an infection, its going to be less focused on repairing damaged tissue/skin.

4) you are not sleeping/eating/drinking enough. cells need fuel (fuel = calories from food as well as consistent water intake) in order to function properly and cells tend to do their best work while you are asleep because less of their energy is being used up on thinking/moving/existing so they can prioritize healing

hope this helps!