Hi Margarita,
If you're using a relatively recent version of iolite 4, just above the data reduction schemes list there is a wrench icon that you can click to find an option to "clear all channels before crunching". By default iolite allows you to accumulate channels when running multiple DRS, but this can be confusing when switching between similar ones (e.g. U-Pb and UComPbine) so we added an option to clear between runs.
That said, if you're using the UComPbine DRS with MAD as the reference material and the 207Pb correction on the reference material, it will appear as a line since the 7/6 ratio has been fixed to the known value.
All the best,