You attend class regularly (perhaps for years), practice at home, and make progress in most poses—except for forward bending. You seem to have hamstrings of steel! No matter how often or how long you practice, there doesn't seem to be any change.
If this sounds like you, it might be because you're looking in the wrong place to find a solution. Hamstrings aren't the only muscle that needs to be worked to see progress in forward bends. A group of muscles in the hip area—the external rotators—can also interfere with the ability to bend forward. Working on poses that loosen up your hips might be exactly what you need to help you surrender into deeper forward bends.