Diahatsu Fourtrak 4x4 help!

Its winter over here in the UK..although its rumoured to be March. I've owned and loved my faithful Fourtrak Rugger for 2 years now, and he usually pulls me out of sticky situations. Twice now on an icey uphill climb I have been left flailing about back wheels spinning, front wheels not helping out! This is despite have my wheelhubs locked 4H button pressed on the dash and 4L lever selected.Can anyone please shed some light, as a friend of mine topped the hill in her Mitsubishi Warrior with great ease, and I fel we need to regain our status!:banghead: