Most of the problems with bed adhesion is because:

• Your printer / gantry is not level
• your bed is not level
• Your bed is not cleaned
• your z-offset is not set correctly
• e-steps not correctly configured

Check out the maintenance page for more info

There could be more issues:

• To much/to little flow
• Printing too fast
• Partial clog in your hotend
• Extruder tension not correct