ajax event not quite working as planned

I have a listview where each row has an expandable section in it to show
additonal detail on some rows. That works. But two aspects aren't working.

1) I am trying to change the + on the button to a -, and then back again
when we collapse. My code looks like:
if(invokingButton.getLabel().getObject().equals("+"))
invokingButton.setLabel(new Model<String>("-"));
else
invokingButton.setLabel(new Model<String>("+"));
target.add(invokingButton);

The label never changes. From stepping in, I can say that yes, the label
set to "-" line does run when expected. yes, outputmarkupId is set to true.

Button code when created:
item.add(new AjaxButton("btnExpander"){
private static final long serialVersionUID = 1L;
public void onSubmit(AjaxRequestTarget target, Form form) {
onActivityLogExpansion(target, this, activity);

}.setLabel(new Model<String>("+"))
.setOutputMarkupId(true).setVisible(showReject || showSubmit)
.setOutputMarkupPlaceholderTag(true));

2) When I go to collapse the row, the event gets to the server, things seem
to work stepping through the code, but then the region does not collapse and
I get the following in the wicket ajax debug:

INFO:
<?xml version="1.0" encoding="UTF-8"?><ajax-response><component
id="expandableWhenReject4c" ></component><component id="btnExpander47"

ERROR: Wicket.Ajax.Call.processComponent: Component with id
[[expandableWhenReject4c]] was not found while trying to perform markup
update. Make sure you called component.setOutputMarkupId(true) on the
component whose markup you are trying to update.
INFO: Response processed successfully.
INFO: refocus last focused component not needed/allowed
INFO: focus set on wicketDebugLink
INFO: focus removed from wicketDebugLink

The container being hidden is created as:
WebMarkupContainer expandableReject = new
WebMarkupContainer("expandableWhenReject", new
Model<ShareActivityPlusDecode>((ShareActivityPlusDecode)
item.getDefaultModel().getObject()));
expandableReject.setVisible(false);
expandableReject.setOutputMarkupId(true);
expandableReject.setOutputMarkupPlaceholderTag(true);

and in the event:

WebMarkupContainer wmc = (WebMarkupContainer)
invokingButton.getParent().get("expandableWhenReject");
wmc.setVisible(!wmc.isVisible());
target.add(wmc);