I would also be adding some Senior Feed to the mix. That is another thing you have to add gradually.
Is this an old horse you are trying to help keep weight on?
I will go through what you might check or things you can do. I blanket my old "kids" with a blanket that includes a neck cover. Medium weight. I would also have the vet check teeth which you probably have done. Have a fecal run just to check to see how your worming program is going.
If the horse is able to chew, I would give all the grass hay the horse will eat without wasting it.
Another thing you might have your vet check for is ulcers. Sometimes horses who are tough keepers actually have ulcers.
Good luck, it's tough to keep weight on some horses.
Don't mess with the old dogs... age and skill will always overcome youth and treachery! Brilliance only comes with age & experience.