Columngrouping in Matrix don't show not existing items

Hello,

I'm trying to create a report in SSRS using a Matrix.

The data in my dataset looks like this one:

Part      Action   SortID  Count
---------------------------------------------------
Login      LA1      1         12
Login      LA2      2         25
Login      LA3      3         548
Register  RA1      1         12
Register  RA2      2          56

I have one rowgroup by the column Part and one columngroup by SortID and my Data should look like this:

Login          LA1       LA2        LA3
                  12         25         548
Register      RA1       RA2
                  12         56

But it looks like

Login          LA1       LA2        LA3
                  12         25         548
Register      RA1       RA2       LA3
                  12         56

Has anyone an idea, how I could solve my Problem, that in row number 2 the LA3 isn't shown?

Thanks for your help