Also, if you are using a shop vac, you might be getting too much air.
If you get the piece close to the grate, a strong blast can actually cool it.
Otherwise, too much air = too much oxygen = mucho scale = no weld.
You need a deep fire, gentle air blast, and patience to let the heat come up gradually.
If it does stick, you might need to work through 2 or 3 (or more) welding heats to close all the gaps. You will need to wire brush and re-flux before each heat.