HELP! My new Quingo scooter hates me.!

Hi all. I am a newbie here but wondered if anyone could offer any help. I purchased a second hand scooter, which on the whole is fabulous. There is just one little...well kinda big problem. The seat keeps coming undone.
I will try to explain. I have a scooter which folds down to go into the car. When you fold it down, you have to release the seat to kind of flip it over. OK.. With me so far? ?? When the scooter is unfolded again, the seat flips back over and is supposed to lock into place. It seems to do this, until it comes to sitting on it. Now, I am not a slim Jim by any means. I obviously checked the maximum rider weight before buying too. See... I was sensible (for once) ?
The seat kinda comes unlocked and tips forward. With me sitting on it! My 8 yo lad finds it hilarious, me, not so much. I was wondering if I'm missing a trick somewhere? Maybe a locking pin or something that I'm not putting in. Or a lever maybe that keeps the seat from doing this. This scooter has given me a new lease of life as I can't walk far. I just think it hates me!! Or I'm too fat maybe and the weight tips it? Rider weight = 25stones max. I'm 17st9.
Sorry for the ramble. Much love. x