this has been discussed numerous times in this forum. The flash will not recognize the batteries and displays a blank battery symbol with a line through it. Some of the fixes have been centered around the adapter for the filter causing an issue. i never use it or have it on the flash. Other fixes suggested are opening and closing the battery door with the unit on, holding the reset buttons, etc. It seems all of these fixes work for some and nothing for others. I had issues with this on a big important shoot with a second shooter and we had to improvise to use my Fashpoint Explorer 300 only, on a light stand. In my humble opinion this is inexcusable! For the cost of these flashes this should not be happening, or Canon should fix or reimburse anyone having this problem. I have inquired with a lawyer re: a class action suit, since Canon seems to plead ignorance to the problem. When you invest in equipment that is not cheap, it should perform as expected and one should not have to fiddle around with battery doors, or filter holders, etc. I bought it to shoot when i need it, not to fumble with. If there is an issue it should have been resolved by Canon long ago.
My immediate fix is to use another brand. The flashpoint r2 recycles faster, is a 3rd the price, doesnt use AA's, and doesnt have any glitches to figure out or play with.