i have try 6 time smelting equip lv 10 +10 to next level but i got 5 filure and that says 80,0% success rate or maybe only me got that **** problem

You just have to pray more to RNGesus.

They use odd random number functions that often generate similar numbers a few times in a row.
For example with a chance of 80%, of 100 numbers, the randomly generated number always needs to be above 20.

So you probably got your random number like this: 12, 16, 8, 7, 18
Just never above 20, so it always failed...

I dunno. Cyberstep is know for having odd RNG number, especially in another certain game >:x (there, I even failed something that had 100% chance, so I take it as a proof, that the displayed numbers are not correct!!! so if for example it displays you a chance of 80%, it actually might be something like 60% only. But thats just assumption, since the thing above with the 100% really happened to me in their other game.)