scala bubbleSort endless recursion

Question

I am trying to get this implementation to work, tho it ends in an infinite loop. What am I missing?

def bubbleSort(l: List[Int]): List[Int] = if(isSorted(l)) l else bubbleSort(bubble(l))

def bubble(l: List[Int]): List[Int] = l match {
  case Nil => List()
  case x::Nil => List(x)
  case x::y::ys => if (x<y) x::bubble(y::ys) else y::bubble(x::ys)
  //case x::xs => if (x<xs.head) bubble(xs) else bubble ()
} 

def isSorted(l: List[Int]): Boolean = l match {
  case Nil => true
  case x::Nil => true
  case x::y::ys => if(y>x) false else isSorted(y::ys)
} 

val l = List(12,35,15,2,1,2,17,51,34)

println(bubble(l))
println(bubbleSort(l))