Training and actual competition are two different areas. In training, you know who you are going to tackle, whereas in competition you are up against the opposition. You might have a subliminal fear that your opponent would like to hurt you, whereas in fact he is just trying to escape from you. Just tackle your opponent in competition as you do in training. After the first tackle your subliminal fears will disappear.
I have had the same problem you have in my first years of playing rugby I had a huge problem tackling but I knew what the problem was it was not in my technique it was all in my head like I was scared to hit people and I was scared to be hit, that might be your problem that was mine but I play prop and a lock now and then so making a tackle was more important so I had to sit down and get my mind right like I had to tell myself that it was ok to hit(tackle) people and I got over the problem of missing tackles, but that might be your problem and it might not be but I hope I helped you out and from one rugby player to another I wish you luck
Chicken
U just need to get low and get them down
