In some packs it's possible to melt items in Tinkers' Construct (not sure about other mods), others it's configured not to allow such processing. In addition, the device to be re-smelted may have to be 100% repaired or it won't smelt.
This is why people need to provide this info about which pack, version of Java and possibly even version of the pack... A beta version may allow re-smelting and later the Dev decides that is too overpowered (OP) and removes the capability before the final release.