Most causes are that your nozzle is too far from the bed! Try adjusting your z-offset.
Your bed maybe too cold, so try raising it by 10 degrees.
The bed surface may be dirty so make sure to clean the bed after every use with 99% IPA.
Lastly, make sure the bed you are printing on is appropriate to use with the filament you are printing. Check out this link to help you choose suitable bed types: What is better to print on? PEI or Glass