bootstrap margin bottom not the same

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CSS

bootstrap

http://oi60.tinypic.com/29lbgad.jpg[^]
as you can see at the first image, each row have the same gap between them.

http://oi57.tinypic.com/2gud1t2.jpg[^]
now let's view at the smaller browser, notice the between different at point 1 and point 2. how to make them the same?
i used both row and form-group, both same result
Code:

  .row {
            margin-bottom: 8px;
        }
<div class="row">
                    <div class="col-sm-2">
                        <label class="control-label">test</label>
                    </div>
                    <div class="col-sm-4">
                        <asp:TextBox CssClass="form-control" runat="server" />
                    </div>
                    <div class="col-sm-2">
                        <label class="control-label">test</label>
                    </div>
                    <div class="col-sm-4">
                        <asp:TextBox CssClass="form-control" runat="server" />
                    </div>
                </div>

                <div class="row">
                    <div class="col-sm-2">
                        <label class="control-label">test</label>
                    </div>
                    <div class="col-sm-10">
                        <asp:TextBox CssClass="form-control" runat="server" />
                    </div>
                </div>

               <div class="form-group">
                        <label for="inputPassword3" class="col-sm-2 control-label">Form</label>
                    <div class="col-lg-4">
                        <asp:TextBox runat="server" CssClass="form-control"/>
                    </div>
                        <label for="inputPassword3" class="col-sm-2 control-label">Test</label>
                    <div class="col-lg-4">
                        <asp:DropDownList runat="server" CssClass="form-control">
                            <asp:ListItem Text="A" />
                            <asp:ListItem Text="B" />
                        </asp:DropDownList>
                    </div>
                </div>