I wrote the following code to create an infinitely scrolling grid, however when i scroll down the postback parameters are always page=1 and start=0, so it only ever displays the first 200 records, what am i missing here?

var store = Ext.create('Ext.data.Store', {
id: 'mystore',
pageSize: 200,
model: 'Proj',
remotesort: true,
buffered: true,
proxy: {
type: 'ajax',
api: {
read: 'Project/DetailsExt',
update: 'Project/UpdateExt'
},
extraParams: {
total: 50000
},
reader: {
type: 'json',
root: 'data',
totalProperty: 'total'
},
writer: writer,
headers: { 'Content-Type': 'application/json; charset=UTF-8' }
},
autoLoad: true
});