Model surface will be too loose in one of the following scenarios
(a) The layer thickness is too big. So, reduce the layer thickness.
(b) Printing speed is too high. Either reduce the printing speed or alter the temperature to maintain faster printing speed.
(c) Printing temperature is too low
(d) Wall is too thin
(e) 3D model isn’t a completely closed solid
(f) Feeding device fixing screw is too loose
(g) Wrong filament diameter is used
(h) Filament quality is poor
(i) Filament gets stuck and couldn’t be fed smoothly.