.save() on existing model causes POST instead of PUT

I'm having some trouble saving changes to individual models in a collection. Models that have been loaded (via a collection .reset()) are issuing a POST (as if they were new) instead of the expected PUT.

Here's the approach I'm taking:

AppView

• Loads the child collection via this.model.childcollection.reset(JSON DATA FROM SERVER);

• In it's render function, creates a new childview for each item in the collection and renders it:

  render: function() {
          var el = this.el;
          this.model.childcollection.forEach(function(s) {
          var view = new ChildView({ model: s });
          el.append(view.render().el);
      });
      return this;
  },
  

ChildView

• In one of its events it is changing some values of the underlying model and calling save:

  this.model.set(
          {
              ValueA: somevalue,
              ValueB: somevalue
          },
          {
              error: function() {
                  console.log("Error saving model");
              },
              success: function() {
                  console.log("Model change saved");
              }
          });
      this.model.save();
  

Observations:

• POST (with no child id) is called instead of PUT (with child Id)
• Child elements have Ids set

Can anyone tell me why this may be happening?