Too flat smashes

Maybe better to have the camera on the side where your racket arm is..

It’s going flat because your racket face is flat when it contacts the shuttle. You need to time the swing so that you make contact with the shuttle when your racket face is tilted down more, so, making contact with it at a later part of your swing.

So you position yourself and do your swing such that when your racket makes contact with the shuttle , your racket face points where you want it to point, in this case, you want your racket face to point more down at the point of contact.

Maybe it was pointed down more when you hit the first one, the cross court one, though that one went in the net. . Cross court shots have to be flatter in order to get over the net. And maybe the fact that you hit that in the net caused you to make the others much flatter to avoid hitting the net.

As to why your other ones were flat. Consider a flat drive. The shuttle travels straight / flat, because the racket is flat on contact. If when you try to do a smash, the racket is flat on contact then it makes for a flat smash.

How all the timings work in order to adjust how steep or flat the shot is, i’m not entirely sure.. but there is absolutely no doubt that your racket is flat on contact, and if you hit the shuttle in the part of the swing where the racket is pointed down more, so at the time in your swing when the forearm is pronated more, then the shuttle will go down more.