You are likely running out of gain and/or base drive on the transistor. Assuming r3 is the 100K resistor you aren't getting a lot of base current flowing. 9v / 100k = around 90 uamps of base current. Assuming a transistor gain of 100 that is about 9ma (or one led worth) of collector current. 9MA will light one led (but if you have 6 of them as shown the same 9ma will flow but likely none of them will light as they are only seeing about 1ma each. You have two choices: reduce the size of the 100k resistor to provide more base current (but that may impact the operation of the cds cell) or add a 2nd transistor with about a 1K resistor for r2 and another 1k resistor in to the base of the second transistor with the leds connected to the collector of the second transistor. That gets you both higher base current drive on the second transistor and another gain of 100 which should provide enough collector current on the second transistor to drive your 6 leds.
Peter Van Epp