Loose blade on Parrot Bebop

Hi, can anyone help with an issue I have please? When I fly my bebop on its first flight all seems well, but on the 2nd flight I go to take off and one of my blades flies off and the drone flips on the floor.

I have checked the mounting of the blade and it works loose everytime after the first flight.

any ideas why this is happening, the screws are tight but it still works loose.

Please help.....