Hi Florence,
If your SNC is active and if the parameter snc/accept_insecure_gui is set to 1 then everything should work fine.
But as in your case, you said it works sometimes and sometimes it doesn't work which is little weird. So can you please confirm if you had set this parameter in the DEFAULT.PFL?
As a workaround you can check the option "Password Logon for SAP GUI permitted" under SNC tab in SU01 for the RFC user and then check.
Regards,
Nirmal.