This usually happens when you first zoom out and frame a large region of the model. I think of it as a focus point for the camera, and the best example is your own eye. If you focus on your finger and move it closer and closer to the eye, you'll loose focus after it reaches a certain minimum distance. Same with the camera in Modeler.
The easiest way to fix it is to select a few of the polys you want to see closer and hit Shift+A to frame them. The "lens" on the camera will change and now you can see smaller details.
Example below.
Of course, this might make larger details unselectable at times, especially long polygons or long edges. You'll have to zoom in and out between detail levels.
I have also seen a minimum scale for zooming in which is different from model to model (for instance 1mm on a 2m model vs 1um on a 20cm model).