That Window Has Lost It's Seal,
It Needs To Be Replaced.
Dual pane windows do not last forever, they will need to be replaced. The insulation value is connected to the space and properties between the two panes of glass, it is sealed. Over time the gaskets will fail and the seal broken.
How do I know if my window has failed?
The easiest and most obvious way to tell is how does the glass look. Once that seal is lost, moisture from outside can begin to get inside and condensate will form on the inside of the panes. This will look like a fog at first, and eventually water droplets and even streaking water on a regular basis.
Some windows tend to last longer than others, contact a qualified contractor in your area for more information.