2nd floor rad was cold today

Fizz

Rad in question wasn't working well as main dipped about 5 degrees, now flattens-out between riser and last feed to first floor, got re-piped it's better and was working great til today. Vent is maid 0 mist #6, brand new,, removed it shook-out residue water, still nothing, then removed vent, steam started moving slowly to rad. Removed #6 cap so open top on vent. Will see if it helps. Noticed lot of movement in rad front to rear. System is single pipe parallel flow.