Saving to user's custom field
It's probably a validation error.  Use this to check if there are any.

$userModel-&gt;getContent()-&gt;setAttributes(array(
    'mycustomfield' =&gt; 'my field value'
));

if (!craft()-&gt;users-&gt;saveUser($userModel))
{
    $errors = $userModel-&gt;getErrors();
}