Hi
If we pass multipul values through parameter then it creates an array and if you place the paramter in your report it will show the first value only. To display all values try to with the following :
Join({?parameter},',')
It will join all your values by separating with comma. If you want to place the first value of your parameter then you need to use split() function.
Ex :
split(join({?parameter{,','),',')[1]
Play around with this functions