Sounds like you were following a bad guide if it didn't advise you on what to meld on each gear piece.

Just like combat gear, crafting and gathering gear have stat budgets that can't be exceeded.

The game also warns you if the materia you are trying to use will exceed the stat budget for the gear piece in addition to letting you know just how much stat will be added before you confirm the meld.

Instead of putting the +10 materia on head, body and hands, put it on earrings, necklace and bracelet since those have a high enough stat budget for CP for you to get the full amount.