Unable to set/get MAPI Properties for single Recurrence Appointment update in Outlook Desktop

Unable to set/get MAPI Properties for single Recurrence Appointment update in Outlook Desktop Application.

While reading the properties we are getting value as null.

Set-

using (MAPI.MAPIPropHandle item = new MAPI.MAPIPropHandle(aitem.MAPIOBJECT))
{
  item.SetPropString(propName, customProps);
}

Get-
using (MAPI.MAPIPropHandle item = new MAPI.MAPIPropHandle(aitem.MAPIOBJECT))
{
  customProps = item.GetPropString(SyncItems.SN_CUSTOMPROPS);
}

We get the value when "The Entire Series" is selected for update but dos not work when you select "Just this one".

Any suggestion on above issue?

Thanks,

Hemant