Well first off, to do a compression test ..all injectors or glowplugs have to be removed to allow engine to spin over fast enough to make for maximum readings ..you'll also need a fully charged good battery
Additional test - In a compression test you can work out ..if rings or valves , by sealing the pistons with engine oil, if, when this is done, compression goes up, it will prove it's not the valves.
Note: these engines have a deep hole in the centre of the piston, so oil should somehow be trickled around the edge of piston ..you don't want to use more than a tea spoon of oil.
Before doing any of this, note that a poor battery or starter motor will prevent engine starting, due to not enough speed to get the compression pressures up. cranking amps (everything back together) on average temperature day should be between 70-90 amps .. anymore and then you suspect the starter . you need a clip on ammeter to do this test.
On another thread here was a guy with 38,000 miles and has experienced the same as you.
viewtopic.php?f=65&t=208956all the best.mark