Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
Seek emergency care for sudden shortness of breath, chest pain, coughing up blood, fainting, or a foot that suddenly becomes cold, pale, blue, numb, or severely painful.
Seek prompt medical advice if you develop new one-sided leg swelling, warmth, redness, or persistent calf pain.
These symptoms need assessment and should not simply be treated with tighter compression.
Seek urgent help if bleeding does not stop or is heavy.
Also arrange review for worsening skin discolouration, a sore that is not healing, or difficulty using the stocking.
The article also says
Contact your healthcare team routinely if the stocking repeatedly slips, rolls, causes a rash, or does not improve symptoms.
If a varicose vein bleeds, lie down, raise the leg, and press firmly over the bleeding point with a clean cloth or dressing.
Every sign above is quoted from the sofidex article on Varicose vein - noninvasive treatment. If you are not sure, err upward — it is always reasonable to be checked.