Custom submit button callback overrides default callback?
Problem seems to be custom form element. Change it to :

$form['custom'] = array( 
    '#name' =&gt; 'custom', 
    '#type' =&gt; 'submit', 
    '#value' =&gt; t('get data'), 
    '#prefix' =&gt; '&lt;div class="button-wrapper"&gt;', 
    '#attributes' =&gt; array('alt' =&gt; t('get data')), 
    '#suffix' =&gt; '&lt;/div&gt;', 
    '#submit' =&gt; array('custom_callback'), 
  );