Well, those couplers are Kadee clones, which are an improvement on the ones that used to come with all train sets (and still come with a lot of them). Check them for height. There is a correct height, but more important is the same height. Put a length of track on a table and look at two cars coupled from the side. The couplers should be level -- none of one coupler showing above the other. The pin that comes down should be higher than the rails by a small amount -- you should see daylight between the end and the rails as you look across (I think .010" is the official clearance.)
Check all your cars and the loco. Double check the ones that keep uncoupling. If you find any that are high or low we'll work on that.
Do you have any switches or crossings? then you have to be fussy about the uncoupling pins. They often need to be bent a little more to be higher. (Also applies to the rerailer)
Do you have uncoupling magnets? I don't know if the sets come with them. If the train jerks a little as it goes over the magnet, it will uncouple.
While you're at it, check that the coupler springs back to center when you push it to the side.
Sorry to go on, but we may get you running before the mail comes back from Seattle.