My folks had mistletoe in their crab apple tree for about 10 years and it never produced berries

But I remember my mum telling me that its only the female plants that produce berries so maybe you've got a male?!

Just found this on a website:
Mistletoe has seperate male and female plants and so you need to have at least one of each in proximity in order to get berries. If you have a single female plant, identifiable by its insignificant late winter flowers, then the alternative is to bring some male flowers from elsewhere and hand pollinate (usually in February or March).