Question about occult cubes

Hi everyone, A few days ago i got an occult cube from root abyss but some people told me that they doesnt work, anyways i used it on a warrior top (with 100%potencial upgrade) and it worked fine i was happy and went from rare to epic you know... so today i bought 2 from fm to cube a thief shoes with 100% potencial upgrade and... now they doesnt work... I did the same thing like the first time, i tried to cube in root abyss and nothing. Im confused anyone got any experiense with this? or what the is happening?