Levelling your bed is essential for prints to stick on the coated glass surface. Compared with PEI, Buildtak, Printbite or even just using hairspray, glue stick or blue painters tape, this coated glass surface is the LEAST sticky of all, and is particularly fussy with getting the first layer height perfectly correct.
In your slicer, it is advised to set your first layer height to be between 0.1mm and 0.25mm. Values outside of this range are likely to cause issues with first layers not sticking, especially if you are using the stock 0.4mm nozzle. More tips on bed adhesion are found in this article.
WARNING: Be careful not to grind the nozzle on the bed - the bed coating is very abrasive and will damage the brass nozzle requiring replacement.
To complicate matters, most of the A5 printers have warped or bowed print beds. You can check this with a good quality metal ruler. Those typically are dead flat, and can be used to test the flatness of your bed. place the ruler on its side on the printer bed, and shine a torch from behind. If you can see light peeking under the ruler, then your bed is not perfectly flat. If you see light near the middle of the ruler, then your bed is “dished”. If you see light at the edges of your ruler, and the ruler can rock slightly, then your bed is “domed”.
A video tutorial on Leveling the Bed can be found here
Unfortunately, if your bed is warped, then there is simply no way to get a flat bed surface, using just the 4 thumb screws provided.
You have a few options at this point: