First of all; please add resistor values to your schematic. It would make everyone's life, who are trying to help you, a little easier.
My two yen:
These classic circuits require an unbalance to start oscillating. In the good old days, transistor's hfe would vary significantly, and would take care of the unbalance automatically.
Nowadays the hfe spread may be tighter. I've seen some designs, which they provide slightly different base resistor values for that purpose.
Having said this............I've built MANY of these circuits over the years, and start oscillating very easily. I don't understand your R5, R6 calculations and ignore why you chose base resistors R5, R6 = 680 ohm. They are at least a couple of orders of magnitude too low for 20 volt operation.