As Tej Parikh (FT) suggests, there's no 'puzzle' to the UK's low productivity, the puzzles is the lack of ability to shape a workable response.
We already know productivity rises are depressed by:
a lack of investment;
widespread failing management;
regional misallocation of what investment there is (too much in the SE);
badly matched education & work opportunities;
staggeringly high energy costs;
and more recently Brexit.
But shaping a workable response... that's the puzzle!