The problem comes about when the plastic of the first layers of your machine cool down thus start shrinking. To fix the problem you need to identify the cause of the problem. One of the ways of fixing the problem is heating the platform in order to keep the plastic at a temperature that is below the point where it solidifies.
This keeps the prints adhered to the bed. Another way of fixing the problem is adding a thin layer of glue to your platform. There are many ways in which you can do it. One of the most effective ways is using rafts which give the print a larger surface area at the base.