I need to know what voltage your HT cable is ? 10 kV 11 kV 33 kV ?
A mimimum of a 5 kV megger is required. a 500 Volts megger hasn't got enough voltage to perhaps jump a gap in the insulation where a void is or a tree has formed in the cable insulation.
An AVO BM 25 is a great Megger for HV cables.
If you have access to VLF set it will let you know if an intermittand fault is present.
A VLF ( Very Low Frequency ) test is good practise to do on a suspect cable or after a repair.
For an 11 kV cable ( 6.3 kV to screen ) the test voltage is peaked to 17 kV at 0.1 Hz test frequency, for a period of 15 minutes, no breakdown shall occur.