The problem in your solution is that you are using "linear probing" for insert operation, but you are not using the same approach for retrieving it.
First of all - I would change your underlining storage to keep whole struct instead of value:
var hasharray [15]Item
Second, I would change the retrieve method to check value of the item with calculated hash index, and after that iterate items one by one to find actual item if there was a collision:
func retrieve(key string) {
    index := hashmethod(key)
    found := false
    for !found {
        item:= hasharray[index];
        if key == item.key {
         found = true;
         fmt.Println(index, item)
        } else if index != size-1 {
            index++
        } else {
            index = 0
        }
    }   
}
See here: https://play.golang.org/p/8JfTpbJcWx