Can not into external break point

Hi experts,

When external system S1 call remote FM F1 in SAP product system, I have set external break point ,and the user is same as interface user, and user type is dialog. But can not debug into FM, and database table has been updated.

When using the same user ID, external system S2 call remote FM F2, external break point is work well.

I am using ECC6.0.

This question nagged me for a long time, can someone tell me why?

Thanks!

Regards