How to fix:
- 
1. Increase the extruder temperature to keep the flow of filament more consistent and allow the material to melt and solidify together with other layers. 
- 
2. Decreasing the printing speed gives the hotend more time to melt the filament which makes bonding between layers better. 
- 
3. A clogged nozzle can also cause separation by decreasing the filament flow rate. Cleaning your nozzle every once in a while should help. 
- 
4. Keeping the layer height low is also known to ensure better layer adhesion, eliminating this issue. 
- 
5. Make sure your filaments are dry.