I sometimes have that problem. My dad for instance and his wife are extremely difficult to buy for. They live in Florida so shipping is an issue and they never want anything but gift cards and I hate to do that. But, I might just do that this year.
My mom is usually blown away by anything that I get for her but, I've really surprised her a couple of years now. One year her washer broke and I bought her a new one for Christmas. That was perfect for her because she didn't know how she was gonna swing getting a new one herself. And then one year we kept her busy while we moved out her living room furniture and moved in her brand new living room set! She was thrilled with that since it was the first new set she had ever had. Those were big years but, it was stuff that she needed and I was able to provide it so I did. Last year she was thrilled that I bought her 500 thread count sheets so I'm getting her 2 sets this year!
It can be tough sometimes but, don't stress over it.