Cast iron baseboard wont heat up

oopsplumber

I have an issue with some cast iron baseboard heat not heating up quickly or evenly. The rest of the house works great. On the 2nd floor of the building someone installed 20' of cast iron base. I replaced the valve and the steam trap. The rad is pitched properly with no sags. The condensate return is clear on the leaving side. The radiator is also not blocked but the steam does not make it across the radiator ending about 1/2 way. All the other radiators are somking hot and this one is 1/2 warm. It is a (2) pipe system fed with 3/4 in and the condenstate return is 1/2 inch. I am kind of thinking that it is just a poor design and the steam cant make it across such a large run.....But I need some input before I make that call...... Any thoughts ??????????