Hi Edafe
The name of your selection group must match the name of your reference material exactly. If it is even slightly different (e.g. different case lettering) it won't be considered an RM by iolite.
You can tell if iolite considers a selection group to be a reference material. In the Time Series View, there is an icon beside each selection group. If it is a baseline group, the icon is an arrow pointing down. If the selection group is reference material, it will have a trophy icon. If it is a sample (unknown) selection group, it will have a target icon beside it. If your reference material has a target icon beside it, you can right-click on it and select 'Group Type -> Reference Material' and it will change to a reference material (asking you which RM it is if it cannot determine it automatically).
I hope this helps.
Kind regards,
Bence