For your "Searchfield", what does your method/function tied to your "search" event look like? What is your function doing?
Are you using FilterOperator and if so, how?
ie. are you doing something like....
aFilter.push(new Filter("ProductID", FilterOperator.Contains, sQuery));
...and straight from the API documentation on Filter...
"It also depends on the model implementation if the filtering is case sensitive or not. See particular model documentation for details."
JsDoc Report - SAP UI development Toolkit for HTML5 - API Reference - sap.ui.model.Filter