LookUpSet is Not working for multiple columns in SSRS 2008 R2

Hi Friends,

I have a problem when i use more than one LookUpSet in SSRS to display more than one value based on Day Number & other fields..Please find the below details and please suggest me a solution for this.

Crystal : 

                Day1                              Day2

Period 4     Arts,Maths               Arts,Maths

For this in SSRS i am using LookUpSet as below based on StaffID,PeriodNumber & DayNumber like  this:

= (IIF(Fields!DayNumber.Value=1,Join(LookupSet(Fields!StaffID.Value & Fields!PeriodNumber.Value &  Fields!DayNumber.Value
    , Fields!StaffID.Value & Fields!PeriodNumber.Value & Fields!DayNumber.Value
    , Fields!ClassCode.Value
    , "TIMSTF5"), "," ),"")) 

which is displaying correct in SSRS,But when i use the same expression to display Day 2 classes it is returning empty even though i have values on Day 2.Please have a look at the SSRS output below:

                Day1                              Day2

Period 4     Arts,Maths              

Day 2 Expression is :

I am not sure why it is not liking to use more than LookUpSet in the same report,Please suggest me with a work around for this.

Thanks in advance,

Sam.