Vue How to trigger vuejs-datepicker after it has been closed

Question

In my vuejs-application I have added the vuejs-datepicker which I trigger by clicking a button. When I've chosen a date, the datepicker is closed but when I try to open the datepicker again, nothing happens and I don't know why?! It seems that it only opens once...

here is a snippet of my code:

<span @click="openDatePicker">open date picker</span>

<datepicker v-model="selected_date" @selected="isSelected" @closed="closeDatePicker" ref="datepicker"></datepicker>

then in my methods:

openDatePicker() {
   this.$refs.datepicker.showCalendar();
},
closeDatePicker(){
  I tried to to:
  this.$refs.datepicker.close();
}

any suggestions?