Power off all of the AirPort Express devices
Open AirPort Utility on your Mac
Click to select the second AirPort Extreme that will be shown under the "main" Extreme
Click Edit in the smaller window that appears
Click the Wireless tab at the top of the window
Make sure that the setting for Network Mode is set to "Create a wireless network" using the same wireless network name, same security and same password as the "main" AirPort Extreme
Click the Network tab at the top of the window
Make sure that the setting for Router Mode is set to "Off (Bridge Mode)"
Click Update at the lower right of the window
Allow a minute for the AirPort Extreme to restart again
Wait a minute
Power up each AirPort Express about a minute apart
Check each AirPort Express using AirPort Utility to make sure that the Network Mode is set to "Create a wireless network" on each AirPort Express.
If you are still having difficulty, post screenshots of AirPort Utility