Strange behavior in Dimensional reporting

[backhandx]

Hi all,
I encounter a huge problem in dimensional reporting.

I have a crosstab with 1 member from L hierarchy in the line, 3 members ( let's say X, Y and Z) from C hierarchy in the column, and a measure.
So far everything works as expected.
When I custom group Y and Z into one column and generate the report, the custom group isn't displayed. I have only the X member in the column.
In the SQL generated by the report, there isn't the Y and Z members in the Clause Where but only X member.
I don't understand why this is happening.
Can anyone give me a hint? is it a problem in the package?

Thanks in advance