wp_schedule_event not fire properly

I have created a plugin, there I use wp_schedule_event. This not not working as I expected.
When I check the schedule by WP-Crontrol plugin it show it is active. But some time if running and some time it is not running.
If I click on Run now then it is working. I have set the interval is 15 min(900) but it is running after few min

add_action(‘cybernetto_inventory_update’,’do_cybernetto_inventory_update’);
if(isset($_POST[‘synchronise_csv’])){
if ( !wp_next_scheduled(‘cybernetto_inventory_update’))
wp_schedule_event( time(), ‘fifteen’, ‘cybernetto_inventory_update’);

}

function do_cybernetto_inventory_update()
{
// schedule work
}

Please help me where I am wrong ?