Why doesn't my adult son call me?

The youngest of my (58f) three adult children (33m, 28f, 27m) does not call/text me at all. My daughter calls at least every other day and I talk to/text with my oldest several times a month. I get so disappointed and sad when I call my youngest, and it goes to vm or send unanswered texts.

Their father & I divorced when my youngest was 5. I feel like I missed out on a lot of time with all of them since we had joint custody. Especially my youngest, since a move separated us, and he lived full time with his Dad when he started high school. His Dad's request. He still lives with his Dad, albeit in his own place on his Dad's property.

We've always been close, he's very loving when we do eventually talk or get together. I just think about how I was with my Mom, there wasn't a day that went by that I didn't pick up the phone, even if just for a minute to say hi and tell her I loved her. Do I continue to call/text or go radio silent (which I've done for about a month now) and wait for him to pick up the phone?

UPDATE: Totally blown away by the traction this post got! Thanks to all for the great advice, shared experiences and thoughts about my situation. I'm happy to share that I went with the suggestion of many and called him just a bit ago and he actually answered this time! I kept it short and sweet since I could tell he was tired (we both work in the medical field), but we made plans for dinner and a movie tomorrow! I'll use the advice that many of you suggested and let him know I'd like to be able to communicate with him once a month and ask if that's ok and if so, what will would work best for him.