AIRPORT Express

Apple designed the "extend" feature to only work among Apple devices, so it is extremely unlikely that this feature would work with routers from other manufacturers....if you are trying to "extend" using wireless only.

Is your "main' wireless router another Apple device?

If yes, what model do you have?

By "extend further", do you mean that you are trying to extend the signal from the existing AirPort Express that is already extending?

Unfortunately, It is not possible to extend from the Express that is already extending.

The new Express will need to connect to the AirPort Extreme to extend. So, it needs to be close enough to the Extreme to get a good strong signal.

Configure the new Express close to the AirPort Extreme to that it will connect correctly. Then, experiment by moving it further away.

You will need to perform a "hard reset" on the Express and then try to configure it again. Do you need help with that?

Make sure that the new Express is powered on and hold in the reset button for 7-8 seconds, then release. The reset button is just above the audio jack as you look at the back of the Express.

Allow about 45 seconds for the Express to restart to a slow, blinking amber light.

Locate the new Express in the same room as the AirPort Extreme to configure it.

Once you have a green light on the new Express, you can move it to an area that is about 1/2 to 2/3 the distance from the AirPort Extreme to the area that needs more wireless coverage.