Here is a quick rundown to where your problem may be coming from:
- not enough filament coming through the extruder. Always, even if you aren't experiencing big problems, make sure that you have clean passageways from time to time. Further explanations can be found in other help sections concerning "Clogging" and in CraftBot videos on youtube.
- not getting enough material in your layers can stem from trying to extrude more filament than the nozzle size is capable. The layer height should always be less than that of the little hole in the nozzle, so don't try printing 300 microns with a 0.25mm nozzle.
- the temperature could be a little off. Often printing problems can occur based on the temp being too hot, especially in the case of a small printing area, but in this case your temperature might be too cold. If the layers are too hot being set on one another you can experience some oozing, but if repeatedly they are too cold, especially if the filament running out of the nozzle is barely hot enough to melt, you may not get the correct layer bonding qualities (think: ABS printing at 200 Celsius -not hot enough!). Turn it up a little.
- the foundation warping based on a not-hot-enough temperature can also be reason for separation, as the surface currently printing is not working on an absolutely flat plane, but rather a on a bowed range. The funny thing about it is that if you also go "way too hot", you can get similar negative results known as "curling". Try your best to respect the characteristics of the material, the physical shape and your printing environment.