If you spot a bad top layer with bucklings or holes and gaps, this is called pillowing. The main Problem is, that the top layer doesn´t overlap with the outline or the infill. In most cases it is caused by less infill, high printing speed or improper cooling.
Most Slicer programs allows to adjust the overlap between the perimeter (outlines) and the infill. The overlap is usually measured in the percentage of how much the infill overlaps with the outline perimeters.
One of the most common overlaps percentages is 15%. If this is to low, try to increase it up to 20% or 30%.
Part cooling fan
If you are printing low viscosity materials or printing with high temperatures, the overhangs between the infill could cause also a problem. To minimize this problem your material has to cool down quite fast after printing it. If your printer has a part cooling fan you should activate it or increase the fan speed. Also check if the fan and ventilation slot are in right position (control, that the air cools your part and doesn´t go anywhere).
Some materials have an extreme low viscosity at printing temperatures and that’s why you must print them with high printing speeds and part cooling fan.
Increase infill density
In most cases of pillowing or incomplete top layer, the infill density is too low. 10-25% are typical infill density values. As always, it depends on your material and printing settings, therefore try out with test cube, to get the best results.
Increase number of top layers
Typically, you set 3-6 top layers, depending on infill density and printing settings. If you have pillowing problems, try out to increase the number of top layers.