Created or shown dimensions?
If created are they saved in the model or the drawing?
This will happen if you use created dimensions and save them in the model then someone over writes your model with one without the dimensions. This typically happens when you have a design department creating models and throwing them over the wall to a drafting department. Drafting department creates all these dimensions in the model but then the design department drops in a new model and the created dimensions are all gone.
That's why I rarely use created dimensions and if I do I create them only in the drawing.