Sometimes the peak or the sharp edges of the print seem to be "melted" into abnormal shapes like following picture shows.
Try resolve this problem by these steps.
1. Lower the printing temperature.
2. Lower the printing speed.
3. Print multiple parts at once.
This problem is caused by overheating. The principle of FDM 3D printing is melting the material, squeeze it out and cool down the paste-like material to form the designated shape. Thus, if the cooling is not fine enough or the temperature is too high, the material will not be able to retain the shape.
Thus you can either lower the printing temperature or printing speed to enhance the cooling, or print multiple parts at once to let each layer has longer cooling time to avoid this problem.