I’d say you probably want a load of flux and to get more heat into those rack mounts rather than the tube. They’ll take much longer to get up to temperature.
If it’s blobbed on and not wetted out then I don’t think it’ll have made a good bond. Give it a light whack and see if it sticks perhaps
Have you got a bronze flux as well as one for silver?
I’d say you probably want a load of flux and to get more heat into those rack mounts rather than the tube. They’ll take much longer to get up to temperature.
If it’s blobbed on and not wetted out then I don’t think it’ll have made a good bond. Give it a light whack and see if it sticks perhaps
Have you got a bronze flux as well as one for silver?