Simple DIAC circuit which I do not understand - LTspice
The DIAC will never reach a current that causes it to properly "flip". As voltage rises across the capacitor, the DIAC progressively steals more current away from the capacitor and you end up with a stalemate - the capacitor can no longer increase its charge (due to the DIAC leeching off a small current): -



You can prove this by removing the capacitor and plotting applied voltage across the DIAC against the current it takes - I bet you that it doesn't reach the break-over point until the current is greater than what the 900 volt supply and 60 Mohm can deliver.