Problem on Acronym exercise

Hi everybody,
I’m stuck with Acronym exercise in Kotlin. I have made this code

object Acronym {
  var acronym = ""
  var nextLetter= ' '
  var charA = 'a'
  fun generate(phrase: String) : String {
    acronym = " "
    // Letter after hyphen(-)
    
    for((index,value) in phrase.withIndex()) {
      if (value == '-') {
        // Get the Next value after hyphen
        nextLetter = phrase[index + 1]
        if(nextLetter.isLowerCase()) {
          acronym += nextLetter.uppercase()
        }

        // Clear nextLetter
        nextLetter = ' '
      }

      // Letter after space
      else if(value == ' ') {
        nextLetter = phrase[index + 1]
        
        if(nextLetter.isLowerCase()) {
          acronym += nextLetter.uppercase()
        }

        // Clear nextLetter
        nextLetter = ' '
      }
     
      // Capitals
      if(value.isUpperCase()) {
        //If the previous letter was caps, don't add to acronym
         if( index > 0 && phrase[index - 1].isUpperCase()) {
            // DO NOTHING
         }
         else { acronym += value }
    }

  }
    return acronym

  }
}

On my computer, it works perfectly and returns me Acronym for all sentences, but in Exercism I can’t validate the test.
I don’t understand why?
Thx in advance. Jack.

Which test(s) fail? What are they testing? What about them fails?