I bought this one:

https://www.amazon.com/dp/B07ZGHCR2M?psc=1&ref=ppx_yo2ov_dt_b_product_details

Its a tougher hose, better connections, and has the gauge attached. Just takes some messing about at Lowes or Home Depot to get a ball valve that fits. To connect to the Mr. Volcano single burner, youll need a ball valve with a 1/4" female and 3/8" flared male connectors. I just grabbed a 3/8s threaded ball valve and used adapters to make it work.

Do you have a meter showing the PSI/bar/kpas that you're putting out to your burner? The default tubes with many burners just have the regulator, and no meter to show how much gas flow you're really putting out. I replaced the default on mine with this one for just that reason.

https://smile.amazon.com/gp/product/B07ZGHCR2M/

With that you'll really know how much you're pumping out. I did a copper melt today and probably overcooked it a bit and I didn't get over 15 psi, which is about half open on a 30 psi regulator obviously. Can you maybe add a photo of the burner and crucible in the furnace? Could be insufficient airflow around the furnace to get it to heat properly too.

These are what I got

Tank side gauge : https://smile.amazon.com/gp/product/B087QZSX96/
Upgrade hose with nice flow meter : https://smile.amazon.com/gp/product/B07ZGHCR2M/