My automatic mechanical watch has gain / loses time, why?

An automatic mechanical watch comes with manual winding mechanism. When the watch is worn on the wrist, the mainspring is wound automatically through normal wrist movement. 

The watch can also be wound up by turning the crown. To do so, unscrew the crown (some watches do not require to unscrew the crown) by turning it counterclockwise.

To start the watch after it stops completely, wind it up either by turning the crown or swinging it from side to side until the second hand starts moving. Then, set the time and date before putting the watch on the wrist.

To wind up the watch, unscrew the crown by turning it counterclockwise, and then turn it clockwise slowly. The watch cannot be wound by turning the crown counterclockwise.

The watch is wound up fully. (Turning the crown further will not break the spring.) Once the watch is wound up fully, it operates for about 50 hours or indicated by your watch.

If the watch is used without being wound up fully, gain or loss of the watch may result. To avoid this, wear the watch for more than 10 hours a day. If the watch is used without wearing on the wrist; If it is used on the desk like a clock, for example, be sure to wind it up fully every day at a fixed time.

If you use a watch that has stopped with the mainspring unwound, winding the mainspring with the crown will not start the watch immediately. That is because the mainspring torque (force) is low at the beginning of its winding due to the characteristics of mechanical watches. The second hand starts to move when a certain degree of strong torque is reached after the mainspring has been wound up. However, swing the watch from side to side to forcibly turn the balance can start the watch sooner.