Carch,
The immune system is greatly impacted by nutrition. If you are not eating a well balanced diet, with some extra protein, your wounds may not heal well.
Unfortunately, certain spider bites contain an enzyme that breaks down tissue. That is how they digest their food. So, this can make it difficult for these to heal. Spider bites take longer to heal because of this.
If you have certain illnesses, such a diabetes, than can also impact on your ability to heal wounds.
Hope your wounds heal up soon. Good luck.