Exclusion of optional attributes makes gml invalid against xsd schema

I have xsd Schema, and gml file according to that schema. When all attributes have values (optional and required) makes gml file valid against schema. But when I intentionally delete few optional attributes on some elements, validator says that gml is not valid against schema.

All optional elements have attribute nullable="true".

What's wrong here?

Example:

Part of schema:

<xsd:complexType name="ADM_DrzavaType" final="#all">
    <xsd:complexContent>
        <xsd:extension base="fgu:KatastarskiGeoObjektType">
            <xsd:sequence>
                <xsd:element name="sifra" type="fgu:CL_ADM_Drzava_Sifra"/>
                <xsd:element name="naziv" type="fgu:CL_ADM_Drzava_Naziv"/>
                <xsd:element name="sluzbena_Povrsina" type="xsd:positiveInteger" nillable="true"/>
                <xsd:element name="povrsina" type="xsd:double" nillable="true"/>
                <xsd:element name="geometrija" type="gml:MultiSurfacePropertyType"/>
            </xsd:sequence>
        </xsd:extension>
    </xsd:complexContent>
</xsd:complexType>

Part of "invalid" gml (sluzbena_Povrsina and povrsina are empty):

<fgu:ADM_Drzava gml:id="Drzava-234">
        <fgu:oid>1122115522125</fgu:oid>
        <fgu:transaction_Time gml:id="Drzava-234-1">
            <gml:beginPosition>20140717190255</gml:beginPosition>
            <gml:endPosition>20140717190255</gml:endPosition>
        </fgu:transaction_Time>
        <fgu:valid_Time gml:id="Drzava-234-2">
            <gml:beginPosition>20140717190255</gml:beginPosition>
            <gml:endPosition>20140717190255</gml:endPosition>
        </fgu:valid_Time>
        <fgu:lineage_Parent>1122115522124</fgu:lineage_Parent>
        <fgu:lineage_Child>1122115522127</fgu:lineage_Child>
        <fgu:napomena>asd</fgu:napomena>
        <fgu:greska>false</fgu:greska>
        <fgu:izvor_Geometrije>GNSS</fgu:izvor_Geometrije>
        <fgu:sifra>BiH</fgu:sifra>
        <fgu:naziv>Bosna i Hercegovina</fgu:naziv>
        <fgu:sluzbena_Povrsina></fgu:sluzbena_Povrsina>
        <fgu:povrsina></fgu:povrsina>
        <fgu:geometrija>
    <!-- this element is long to much, so I will omit it. -->
        </fgu:geometrija>
</fgu:ADM_Drzava>