Someone told me of his unsuccessful attempts to reach a business contact by phone. Despite leaving frequent messages, this other person wasn't calling back. The caller was pretty confident he'd done nothing to cause offence, so he wondered aloud if it might mean that this man could have been suffering from the dreaded blues, and was therefore shutting himself away. Having been there and done this myself, I had to agree that it was a possibility.
It's easy to get to a point where you've had as much as you can take from the world around you, so you shut yourself away. The trouble is, it can be easy for friends to feel slighted if you appear to be avoiding them. Well you are, aren't you?
If you were the one with the low mood, what would you want someone else to do in such circumstances? Well if they had the persistence, I'd want them to keep calling. I'd hope they wouldn't accuse me of being difficult. I'd love them to be patient with me. Maybe you'd feel the same? If so, it might be worth remembering this when someone doesn't return your calls or messages. Try not to storm off in a huff. Just keep leaving those messages. They could well be in a difficult place, and your patience will almost certainly mean a lot to them.