First, check if your time and weather widget are taking too much space.
To check: long press on that particular widget and you will see how much of space it is taking.
If you are using two different widgets for time and weather, it's recommended to use one by choosing it from:
- Tap Widgets from lower settings appearing on your screen,
- Right swipe widgets to see more,
- Choose Weather from widgets and choose widget as I am using in above pic, and it uses less space.
You can also change your App screen grid by going to Home Screen Settings on the lower right of the screen (as appears in your uploaded picture).
Now, choose from menu.
To add new shortcuts it is very useful that you do it by long pressing on any app from App drawer, dragging it and releasing it whereever you want to place that app.
Note: you can only put apps on the blank space of your Home Screen. To check space you have to long press widgets (that are on your home screen, such as Clock, Weather widget, Google Search, etc.)