What's "it"?

Here is a ref for the IIF() statement: https://msdn.microsoft.com/en-us/library/ms157328.aspx

I'm not sure what values aren't coming up as you expect, but if you have a problem with TC.Value not being returned correctly?, you can use this:

=IIF( ISNothing(Fields!TC.Value), 0, SUM(Fields!TC.Value AND Fields!Den.Value)+SUM(Fields!TCO.Value AND Fields!Den.Value)+SUM(Fields!TCCF.Value AND Fields!Den.Value))

The way this works is you use an

=IIF(test condition, return if true, return if false)

Add other IIF expressions to test your values. Or use an AND with a few to ensure that all your return values are present.