Check the hotend is close enough to the platform when printing the first layer. The nozzle should be squishing the filament down.In Cura reduce the first layer print speed to 25mm/s.Make sure that the surface is free of any dust/grease - use an alcohol wipe to be sure.Turning on the brim or raft setting in Cura will also help to create a strong first layer to print onto.Ensure that there isn't any strong draft blowing over the printer that would cause the first layer to cool too quickly and pop off.If you're still struggling you could try applying a layer of pritt stick (paper glue) or hair spray to the bed for better adhesion.