Getting prints to stick to your printing surface is often the biggest hurdle people have when starting out with 3D printing - especially with the A5 and A3S printers. Here are few essential points to note.
Don't forget to make sure you're printing on a clean surface each and every time - wipe down once with pure water, then once with ethanol, isopropyl alcohol, or methylated spirits (metho). (Avoid acetone) This will remove all grease from the surface.
You may need to try several prints and clean after each one to get plastic to properly stick.
Keep the speed for your first layer slow, to prevent parts being pulled off the bed.
A first layer speed of 10-12mm/s is a good starting point.
The first time you print, you may want to set the print bed manually to the desired temperature (67C for pla), and wait 5 mins, in order to give the heat time to fully soak through the glass bed.
Regardless of whether you're printing with a coarse or fine layer height, you want the initial layer height of the initial layer to be set between 0.1mm to 0.15mm. This is because the ratio of nozzle diameter to first layer height needs to be 3:1 or 4:1 to help material be properly squished onto the bed.
If the print is still not sticking, try loosening all four levelling thumb screws by 1/8th of a turn, and try again. This will reduce the first layer height very slightly. Repeat this until the print starts to stick well.
You may need to use mesh bed levelling to get the first layer height perfect across the whole bed surface.
In most slicers there is the option to turn down the fan speed just for the first few layers that are printed. This may be good to try if your prints are curling in the first few layers.