If the seats are really hammered, you might have to go to much more powerful products.
Let me give you an example. 2 weeks ago I did a deluxe interior cleaning for a client. This is my highest package where I spend about 5 to 6 hours just on the interior. His car was a 2006 Volkswagen Passat and his beige leather seats had never been cleaned.
I always start with a 10:1 Meguiar's APC+ cleaning using a soft brush on dual action polisher. After the cleaning was done, I could barelly see a difference between the cleaned seat an the dirty one. So I went at it again using a hand brush and applying pressure. Again, no real difference. I switched to Meguiar's Super Degreaser 4:1 with a hand brush. This time I saw a major improvement but the seat was still dirty. I could see uneven colors in the leather and there were clearly dirty spots. So I switched again to an orange degreaser. This time the seat came perfectly clean, even on parts I had not touched before.
After wiping everything down with a damp microfiber towel to remove all the degreaser, I applied Meguiar's Gold Class Leather cleaner and conditionner. The seats looked brand new.
Leather is coated with a clearcoat similar to what you have on the paint. It's extremelly durable. Leather cleaners are designed to clean delicate uncoated leather, so they are too weak for any real cleaning. Usually an APC will do a good job, but sometimes they are simply too weak for very heavy dirt and grime from years of use on leather.
Here are the pictures:
Vokkswagen Passat 2006