The IF/ENDIF block isn't failing. xTaskbarX is already defined, so it's discarding the next one found. If for some reason you want to use a variable in the xTaskbarX definition, then use a different name.

SomeOtherName 100
IF volume
xTaskbarx $100+SomeOtherName$
ELSE
xTaskbarx $200+SomeOtherName$
ENDIF

But, if SomeOtherName is going to be static, I don't see the point in it in the first place.