Proteus BJT Astable Multivibrator Simulation Error
Two suggestions....

Place a small valued capacitor from one of the transistor bases to GND. This will imbalance the circuit so that it will start to oscillate at startup. 

Simulations are often too prefect in behavior and the two sides of the vibrator will come up balanced. In a real world this would be much less likely becasue the two transistors, two capacitors and the resistors would have a natural variation of characteristics that cause the circuit to imbalance at startup. 

Secondly do consider moving your LEDs to be in the collector circuits instead of the emitter circuits.

Edit:
I would also suggest changing R3 and R4 to 10K to start with. I found that with the 2.2nF capacitor on one BASE and with the above changes that the simulation stated to toggle. Note I also made the 9V supply switch in with a rise time.