PPPOE server does not work for all XP clients on LAN

Hi Ive got a switch to which a mikrotik server with pppoe-server is connectd and another 12 XP clients …4 of those XP clients can not authenicate to the pppoe-server although I did not setup any particular restrictions…did I miss somethin here??
Any suggestons ??
Thx for any help ..

authenticate??

authenticate means that XP is detecting the pppoe server and service is available but username & password its rejected(typically 691 error).

what are you using for pppoe authentication? radius?

Asad