Lack of interesting/challenging projects that come with a air of freshness every 2 years.

This never gets discussed enough. But if there is some thing else more interesting than your day work, your mind wanders off to the next easily accessible interesting thing. In most cases that is HN/Reddit/whatever. The problem may not be HN itself, but a rather more fundamental thing than that.

Every time I've changed jobs or projects, I've seen a sure increase in my productivity.

There fore if your productivity is just flat. Its probably time to check if you have a good enough project to work on, or check if there is other stuff like office politics and a unfair work environment.

Very rarely have I been in a situation where my productivity has fallen due to things apart from these. Even if it does, simple work arounds generally come handy.