LTC7000A not switching at lower voltages

LTC7000A datasheet suggests that it works for the wide input range of 3.5V to 135V Vin. But in my application, I am facing an issue with the MOSFET switching at lower voltages. In my design I have four of the driver and MOSFET circuits, two 12V and two 5V. The circuits with the 12V are working fine but the 5V circuits are not switching. I need to control Power supplies to 4 different loads ranging from 1A to 6A. I have observed the voltages on BST (9.18V w.r.to GND) and TS (1.62V w.r.to GND). Tried changing the BST-TS capacitor but the output is still the same. Tried replacing the MOSFETs and LTC7000A chips with the new ones suspecting the damage, but still the same outcome. The Voltage on VCC pin is 4.5V, I also tried shorting this pin to Vin 5V suspecting the lower voltage. Could someone suggest where I am doing wrong and guide me to resolve this issue? I am attaching the schematic file for reference.