How to correct my backspin serve

Your first bounce is too close to the net which makes the ball can’t get over. Try to adjust the placement nearer to your body and lower the contact point of racket and ball to keep it short and low. 

In addition, you use penhold grip which is a very strong advantage for serving and short ball attack. Try to exploit all that. Hope this helps. 

Your backswing has no wrist whip at the end of the swing as you pause the swing. To change this try starting the backswing as the ball is dropping and not before. Look at the pros serve to see how long they wait before starting the backswing. If you get more whip the bat speed will increase and the spin will go up a lot.

Hey Jeffrey, there are a couple of things to think about that will really improve your backspin service, I don’t think it’s far away from being a very effective serve. So first off is something that had been mentioned which is the first bounce on your side, sometimes this is far to close to the net meaning it lands straight in the net so aim for halfway up your side of the table. Second is your bat angle, you get a lot of spin because your bat is flat or even tilted upwards when you contact the ball, but often it’s not consistent, so try with a slightly less flat angle and you should definitely see improvement in consistency, when you do you can work on getting the angle for the most spin possible. 

Thanks for the feedback guys.
I do understand that the first bounce cannot be too close to the net, but I seem to have difficulty controlling it to be closer to myself.
I will try with the bat being less flat, but I wanted it to be flat so that the serve can be short and that I can brush the ball more instead of more hitting.
I will experiment and see how it goes.
Cheers.