Take a look at this guide, step 9 (particularly picture #2).
First and as far as I can see, the ribbon cables have notches on both sides and have relatively long contact blades. So, first carefully push them into the connector in a 30 degree angle. When you finally feel a resistance, they are "docked" in the connector. Now, before you flip down the flaps be sure that each cable lies flat on the contact bar and is fixed on both sides through the notches (use the image metadata of picture 2 to enlarge it so that you can see more detailed). Each ribbon cable has to lie "in" these notches.
I hope this works. But in case not, I would try to lock the flaps without the ribbon cables and see if they still work. Always push these laps down all along their length, e.g. through a flat metal spudger, pencil or short ruler.