Edge loop stops and doesn't go around the whole object

Question

Please see this picture:

I made a gun scope, and I need to add some edge loop for sharpen edge while applying subdivision modifier.

But problem is, you saw the picture, edge isn't connected, so I can't add edge loops anymore.

This is another angle of the model:

I don't know what causes this, but sometimes it occurs and this is not a first time.

I want to know why this happening, and also want to know how to fix this.

Anyone if knows about this, please give me some advice.

Thanks!