Why every time when I make some changes to the models the dimensions I created in drafting environment disappear?
Also the tables disappear altogether.
In your customer defaults go to Drafting>General>Retained Annotations and make sure the "Retain annotations" box is checked. This way, dimensions that lose associativity will not be deleted. Instead they will change colors to let you know they are no longer associated to the model.
I checked and found that the "Retain annotations "is actually checked.
It turns out that this setting is also saved with the part file and over rides your customer defaults when loading the part file. In Drafting, go to Preferences>Drafting>Annotation and see if it is checked there also. If not, check it and save the file. Also open your default part template and make sure it is checked there and save the part. If not, any new parts you create will not retain unassociated dimensions.