Uneven heating in some Cast Iron Radiators

I have an old 2 story home with a boiler and cast iron radiators. I only have 1 zone, but the supply splits up into two loops. I believe that the loops are not divided by floor, but by North half/South half. 3 of the radiators are not as hot as the others. I think that colder radiators are at the tail end of the loops. Curiously, the return pipe is slightly hotter than the 3 lukewarm radiators....

I have 1 circulator pump.

Any ideas?