Can I know which location you are changing in your parts? If you detail that, I will try to help you out.
Because, if you change almost all the part obviously the dimensions will get removed. So, please be detailed about your work on it.
Thanks and Regards
Derived sketch has a new internal ID. This is the reason why in a drawing all dimensions must be recreated again.
If instead of use derived sketch configure dimensions on the same sketch, all basic dims will be visible.
Normally you will only lose all dimensions if you change the view (example front to back) or as Naga stated the model is changed drastically. Would be extremely helpful if you could post one of your models.