knockout model - observable empty in IE 11 - asp.net-mvc-4

Question

I have a set of input fields that are used to set search params, passed to the controller via knockout and ajax upon pressing the 'search' button.
Each input is bound to a property in a viewModel - each one is a ko.observable().
I have code bound to the keyup event for these fields that will invoke the same search operation when the return key is pressed.
In Chrome, this works fine, but in IE(11) this is never passed!
I have also noted that if I press the tab key to go to the next field, and THEN press return, the search parameter I expect is now populated.
Any ideas? at my wits end...
How do I do the same as pressing the tab key in code, without pressing the tab key?
I think IE's handling of the .change() event in jquery might be different to everyone else...
edited to include sample code - js:
var searchVm = function () {
var self = this;
self.reference = ko.observable("");
self.postCode = ko.observable("");
self.description = ko.observable("");
self.doSearch = function () {
var date = {
Ref: self.reference(),
PostCode: self.postCode(),
Desc: self.description()
};
// do the ajax call to controller
// ... etc ...
}
}
$('.searchField').keyup(function(e) {
if (e.which===13) {
$('#btnSearch').click();
}
});
the page:
<div class='indexRow'>
<label>Reference:</label>
<input type='text' class='searchField' data-bind="value: reference" />
</div>
<div class='indexRow'>
<label>PostCode:</label>
<input type='text' class='searchField' data-bind="value: postCode" />
</div>
<div class='indexRow'>
<label>Description:</label>
<input type='text' class='searchField' data-bind="value: description" />
</div>
<button data-bind='click: doSearch'>Search</button>