Modem console connection time-out

Dear Sirs,
I'm faced with a problem with 5 of our Linkstar S2A modems. When trying to connect to the modems through console the connection times out thus i can not save any bootparameters. I use either putty or SecureCRT to telnet into the modems with their default IP addresses. Using the same setup on other modems just works fine.
Has anyone encountered this problem? If yes, how did you solve it?