I have a hot water boiler system in my house. Overall the house is nice and warm. Upstairs Bedroom 1 has a nice big old cast radiator with a line coming pretty much straight up from the boiler to feed it. That room is fine. The rooms that are not warm are highlighted in dark blue. They all have convectors (two are only one year old). They just don't seem to warm up. The system is only one zone, and I'm thinking that is the problem. Last night I cranked the temp up to 73 and it was warmer upstairs, but the convectors still didn't seem that warm. So I went and checked the lines coming upstairs (in attic space). The line coming up was hot, but the return was barely warm.

Any suggestions/observations based on my drawing of the system? Just trying to see what I'm looking at here and what options we might have.