What can I do to effectively remove grease stains from a sweatshirt that has already been washed and ironed with brown paper, yet the stains persist?

Remember that each garment is different, and some fabrics may not react well to certain cleaning methods. Always adhere to the care instructions provided for your sweatshirt to avoid any unintended damage.
Grease stains can be a nuisance, particularly when they've been subjected to washing and heat, which tend to set them into fabric. However, by following the above steps and being diligent in your efforts, even the most determined stains can generally be removed, returning your sweatshirt to its former glory.