troubleshooting advices are welcome, openbsc website
was no help
The clock of the non working one might just be too off ...
See
http://openbsc.osmocom.org/trac/wiki/isdnsync for a tool to
calibrate (with appropriate HW).
If you don't have the equipement ... huh, good luck.
You can always try random values until it works.
If you have a USRP you can try to lock onto it and compare offset ...
Cheers,
Sylvain