I have it now twice a day, it seems it runs only once a day after midnight, that is strange and it works only for 3-4 days then it stop working completely.
Can you try the lower values, like 5 minutes or so? If there is something regularly clearing the cron jobs, that might prevent that.
Sure I can try it but what happens if first cron indexing will last for 20 minutes or so? And another cron is in 5 minutes?