Growing consensus among economists is that productivity slowdown is key to understanding stagnation. The top commenter almost put his finger on it when he said

Growing consensus among economists is that productivity slowdown is key to understanding stagnation. The top commenter almost put his finger on it when he said that workers aren't paid more because they aren't worth more, before attributing it to the "monopoly of capitalism". In reality if the machines we rely on to be productive aren't improving much themselves, then our productivity is capped. Technological innovation has slowed down a lot over the past 20-30 years, and is now diffusing throughout the world rather than being pushed forward by the large industrial conglomerates, pharmaceuticals, chipmakers, OEMs, etc.