Melding Question. Please help.

I'm having a hard time melding materia, if someone could be so kind to help, i'm sure more will benefit from it.
Right now I'm a lvl 27 CNJ and a lvl 27 Weaver. Crafted my self a lvl 27 robes and currently I'm trying to meld a Mind I materia into it's slots.
However, MInd I appears as too low level to meld it in it, while Spellcasting, for example, would fit just fine (just the first socket). Why is that? Both materia are lvl 15 and next tier of materia is lvl 30! if I meld a Mind I materia into it, it tells me I'll get zero benefit.

There are caps on how much of one stat can go on an equip, and the default stats on that equip also count towards that cap. So yes, if the equip you're trying to meld a Mind materia into already has Mind stat on it, it's entirely possible there will be no effect. Try spreading out your useful stats, in this case meld spell speed or Determination or something else that would benefit you.

Thank you for your reply!
Is there a way to know an item status cap? It seems so random...

There is but you have to have both the gear and the materia on your person. So for instance you have a str III materia which is +4 anda cobalt belt nq. You cna go to the meld screen on any class and select both the belt and the materia and you will see the success rate as well as the stat amount that will be given. So cobalt belt has 5 str already on it the materia stat increase will show in red (which means not total bonus is given) adding only +1 str so the stat cap for str on a cobalt belt is 6str. Normally on similar stats like vit and dex the stat cap will be the same but parry stat cap is +10. Now if these is true of all belts at that level I do not know. But at least for cobalt i know and I do not have to waste te str III.