Like the others have said, definitely rotate spots. You could have a build up of scar tissue in the area and the irritation from frequent injections in the same area is just making it worse. You also may not be allowing the oil time to clear the area before you pin again. The lats and ventrogluteal are both really good, large muscle groups to pin and can tolerate a larger volume of oil. The spot injections website or YouTube are both good places to find diagrams and videos of where to landmark each muscle group.
M18
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