Ethernet on BBB only working after reset not after power up


the setup is:

Beaglebone Black (A5C) connected to a LCD7 (A2) with Linux kernel 3.8.13.

Power is fet into the jack of the LCD7 (mains adaptor has 2.8 A).

Pressing the power button of the LCD7 starts the LCD and the BBB. The
boot process ends in the login prompt (console).

Due to other problems with the USB/USB-hub I cannot login with the
attached keyboard but need to use ssh.

Neither ssh nor ping nor other tools in this context see the BBB.

After pressing reset on the LCD7 the ethernet connection establishes
and ssh login works.

How can I make ethernet work even after a power on ?

Thank you very much for any help in advance!

Best regards,