Layer separation is when there is poor adhesion between the layers in your print. In most cases you can fix layer separation issues by adjusting settings like the temperature, print speed or layer height.
Adjust The Temperature
Temperature settings can be adjusted at the print head (extruder) or print bed. If the print head temperature is too low, this can cause poor lamination between the layers in the print. In this situation you would then experiment with a higher extrusion temperature.
A low print bed temperature can cause warping due to poor first layer adhesion. Again, here you should try to use a higher temperature setting.
Modify Print speed
It is common for creators who want to complete their prints quicker to increase their print speed. The problem with increasing your print speed is it can affect the extrusion of material which can create adhesion problems. Slowing down print speed will almost always improve the quality of a print.
Change Layer height
Layer height is the height of each layer in a 3D print. It can affect not just adhesion between layers but the detail in your prints. Thinner layers give better adhesion and detail to your prints. They can also improve the strength of a 3D printed part.
The biggest downside to printing thinner layers is that this increases the time it takes to print. Source