TPS54233: Doesn't start if Input is below 4V

Part Number:TPS54233

Hello, 

I have made 3.3V power supply using a TPS54233, It works fine if Vin or BATT_LIION_OUT is above 4.3V. 

But when VIN or BATT_LIION_OUT is below 4.3V, it stops working, whereas according to the datasheet the component should work with Vin as low as 3.5V.

I have trie dwith lower values on C6 and C7, but the result is the same.

Could you help me to solve this problem please ?

Thank you in advance.

R1 and R5 are not connected.