Sometimes what seems like a lack of gratitude is actually low self worth. A refusal to receive good things, good treatment, because it's felt undeserved.
If you're being really kind and or generous to someone and they just don't seem to care, don't take it personally or think your best isn't good enough...it may be that it's too good. Upping your efforts may even be met with total sabotage because the person feels so much inner pressure of unworthiness.
While you can't up someone else's self worth, you can direct your efforts to other places where they are better received. That will take the pressure off the one who is unable to receive and allow you to remain your kindest, most loving self.