When you assign a training plan to your client, you should be able to find it in the client's activity calendar. If you cannot find it there, it might be possible that you have assigned the plan while not connected to your client, which means the plan is now in your personal activity calendar. You can check this by switching your account to "personal" in the top right corner of the screen, and check your activity calendar.
Make sure you are connected to your client before assigning a training plan. This means, you will have to go the Coach app first, instead of going to the Training app directly. Confirmation of being connected is the black bar at the top that has information about your client on it.
If you still cannot see the training plan in the activity calendar of your client, make sure the training plan contains exercises. An empty plan can be added to your client's activity calendar, but it will have nothing to display so it will look like nothing happened. You can check this by editing the plan in the plan editor.