cause code ‘ms_lost’ at SGSN

hey guys, i have a query related to a cause code (at SGSN end) which is overall showing relatively high %age in our network (2%), cause code is ‘ms_lost’ and its only available description is ‘radio contact lost with the MS’… no more details. Anyone experienced this in their network? what could be the possibilities for this cause code at SGSN side? this cause code gets triggered when SGSN do not get any response from MS in the attach procedure and SGSN is not getting any response, now i have no idea why…..