Combo box do not select Value from drop down

I am using ExtJs to create a combobox.

Here is my code:

Code:

{
        xtype : 'combo',
        id: 'createformTypeCombo',
        allowBlank: false,
        forceSelection: true,
        editable:false,
        store: formTypeStore,
        valueField: 'value',
        displayField: 'name',
        width: 260,
    }

And here is the store:

Code:

var formTypeStore = SailPoint.Store.createRestStore({
            url : SailPoint.getRelativeUrl('/rest/form/formTypes'),
            root: 'objects',
            fields: ['name','value']
    });

It works fine and combo box is displayed and I can select value.

But if I open and close it 4 times, then in next show it shows the values but It do not allow me to select the last two value. I can only select first value.

Please suggest.

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**Note:** I have tried copying and executing this code in forms of other pages of my application. But behavior is same in all cases.

Combo box do not select Value from drop down