For auto TTL compatible flash either use olympus own brand which is pricy or Nissen i40
which is affordable. Yongnuo does not offer TTL in M43 mount only canon or nikon but you can use full manual flash YN560 mk4 or YN660 which can zoom to 200mm (most powerful) on all brands as it uses only center pin.Most people eventually dump the manual one for TTL,answer is it's not intuitive to use if you don't know the theory of flash usage plus if only single flash it's takes the guess work out
and let's you concentrate on taking photos rather than figuring out the setting for exposure.
For off camera radio wireless TTL trigger for ttl try cactus V6 mk2 but check if it support M43 (nissen i40) which I think it does but may not be 100% compatible.
Whatever brand,use a fresh set or another brand to check eg. sony,eveready or better still alkaline type which last longer for more flashes.If you have a digital multimeter measure the battery voltage if it's less than 1.4 volts (fresh will read 1.5V - 1.6V, nomal rated voltage is 1.5V for one use type and 1.2V for rechargeable Nmh (nickle metal hydride)
the battery has been drained somewhat so may take longer time to charge or flash will not charge up at all.You will hear a high pitch noise/sound like eeee...this indicates flash is being charged up.
Depending on the design of flash electronic circuit. Nissin may have a low voltage cut off point in which if it's below a certain voltage it will not operate hence indicating user to change to fresh set of batteries.Barring the low voltage lockout there could be other component failure as there is the high voltage convertor which takes the low battery voltage (6 volts) and boost it to depending
on the xenon flash tube trigger voltage about near to 300 volts. If it's still under warranty you could send it in for repair but check with a fresh set of batteries first!