Hi mate,
I received the same watch a few days ago and love it, other than the clasp which had the same problem as yours. After a bit of thinking I've managed to fix it and its now perfect.
If you have a look on the inside of the clasp you will see the 2 lugs coming through a plate held on with 2 small screws, this plate is what is causing the problem. You need to open up the slots in this plate slightly (towards the outside edge) in order to allow the lugs to move out further and engage.
To do this you need to split the bracelet, (I use an upside down drill in a pillar drill to press a pin through) & unscrew the plate and lift it out. You will then need a needle file that fits in the slot, I ground one to fit the exact size as its easier to keep flat, you don't need to take much out, just keep filling and trying it every now and then until it works as it should. Simple! Reassemble and Roberts your fathers brother, no more worrying about the thing falling of in the middle of the pub!
One last thing to be aware of is if the 2 small screws in the plate don't screw ALL the way in the heads can foul the clasp and also cause it not to perform, make sure they are flush to the plate, run a needle file over the heads if you have to.
Cheers,
Justin.