My daughter did the same thing--slept through the night with no problem, but would not nap, no matter what I tried. I tried letting her cry (she would cry for hours, and never fall asleep), sitting in her room with her, but not looking at her, all of it. I even got in her crib with her once. When we were getting her to sleep through the night, letting her cry with periodic visits worked, but it didn't work during the day (and we did have room darkening shades in her bedroom!) So eventually, I just surrendered. At naptime, I would put a book, glass of water, tv remote, and phone next to my rocking chair, and just let her nap while I held her. She still slept through the night with no problem, and she got good naps, which helped both of us. And, now that she's 8, I still treasure the memory of those hours of holding her, watching her sleep, and enjoying that closeness.
By the way, as she got older, and was out of her crib, I would put her for naps in her bed--my rule was that "you don't have to sleep, but you do have to lay quietly and close your eyes." More often than not, she would fall asleep. She and I still joke about that little naptime trick--she said that someday she's going to use that one on her kids!
Good luck!