new field in tca.php stores value in DB but not in flexform

i have extended an exisiting extension in typo3 using the tca.php of the extension and created the structure in the DB. Everything fine so far and the field is available in typo3 backend. I can fill the checkboxes and the values are stored in DB but not in the flexform. Any idea why the flexform does not show the values from DB?

'panelshapes' => array(
        'label' => 'LLL:EXT:'.TT_PRODUCTS_EXT.'/locallang_db.xml:tt_products.panelshapes',
            'config' => array(
                'type' => 'select',
                'size' => 5,
                'renderMode' => 'checkbox',
                'minitems' => 0,
                'maxitems' => 5,
                'enableMultiSelectFilterTextfield' => TRUE,
                'items' => array (
                    array('Gerade', ' Gerade', '',' Gerade'),
                    array('Durchsicht', ' Durchsicht', '',' Durchsicht'),
                    array('U-Form',  ' U-Form', '',' U-Form'),
                    array('L-Form',  ' L-Form', '',' L-Form'),
                    array('Rund',  ' Rund', '',' Rund'),
                    array('Prisma', ' Prisma', '',' Prisma'),
                ),
            ),
        ),