Monostable Transistor Circuit not stable
R6 must supply enough current to saturate Q1 fully with all the current flowing through R4. It has to have less than about 0.5V Vce or Q2 will start to turn on, preferably more like 0.1V. So you should have about a 20:1 ratio of currents for most transistors (forced beta of 20). 

The collector current of Q1 when on is about 5mA, and the current through R6 is about (5-0.6)/100K = 43uA (consider the capacitor an open circuit for this analysis). That's a ratio of more than 100:1. 

Q1 probably has an hFE of well over 100 with a significant Vce, but the effective current gain drops as the transistor saturates and you need it to be well saturated or Q2 starts to turn on.