Hi,
To understand the reason you can go and check the Account Dimension properly
You have a Rate Type property to be maintained for both Balance Sheet and P&L Accounts.This ratetype is referred in business rules.
So it could be either place missing - either you have missed out to maintain the ratetype or your business rule is not calling that ratetype which you have maintained for P&L Accounts.
3rd reason could be what kalyan pointed out --missing exchange rates for the period with rate type used for P&L Accounts.
Regards,
Rakesh