What batteries are you using: Alkaline, Ni-MH, or Lithium? When you replace them do you replace the full set? This is important with rechargeable Ni-MH & Lithium batteries. I mark each battery with a set ID so I always Use & charge them together.
Try a fresh brand new set of Alkaline batteries (4). Does it charge up? Does the flash test button fire off the flash?
If it does then the rechargeable batteries you are using are bad and need replacing.