Getting prints to stick to your printing surface is often the biggest hurdle people have when starting out with 3D printing. Here are few essential points to note.

See this article.

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 or methylated spirits (metho). Avoid acetone. This will remove all grease from the surface.

Keep the speed for your first layer slow, to prevent parts being pulled off the bed.

10-15mm/s is a good starting point for the first layer speed.

The first time you print, you may want to set the print bed manually to the desired temperature (60-70C 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 layer height of the initial layer to be set between 0.1mm to 0.25mm, presuming you are using a sheet of paper (~0.07mm typical thickness) to level the first layer. This may require you adjusting the “initial layer height” setting in your slicer to compensate. This slight over-extrusion can help to provide some additional pressure pushing the filament 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.