you physically can't meld it or it's saying +0??

If it's saying +0 it means whatever you're trying to add is capped. For instance if you're adding a +1 control materia to it and it's saying +0 that means its capped on Control and you'll want to move onto Cp or Craftsmanship.

If it won't let you add another materia for some reason even though it's not capped then thats a bug and should be reported.