Problems charging digi-q tanks

Hi, Ive got a couple of digi-q tanks which Im having trouble charging and wonder if anyone else has had this problem.

Ive followed the correct sequence - tanks off charger set to charge insert tank switch controller on but instead of a constant CH in the display both tanks have a flashing CH and busy light.

Im using alkaline batteries so that shouldnt be an issue and everything seems normal with the controller apart from this, any ideas?

Thanks