The reason for this issue is the gap between the nozzle and the print bed is too wide. Make sure your print bed is leveled correctly. Reduce the height of the very first layer – depending on your printer and the filament used, you may go down to 0.1mm (this, however, requires that the print bed is absolutely level). In Cura Software you could increase / decrease the Shell Thickness and the Infill Density to check whether the lines are visible at the bottom layers by printing a small cube first.