Sounds like it's a Shimano compatible hub not an actual Shimano hub. Probably a cheap one (that's why it's breaking.) If you can even find parts for this they will eventually fail too. I've never had a Shimano cassette hub fail, though I've heard of it happening before.
I'd recommend a wheel with a Shimano hub or rebuild your current wheel with a Shimano hub if the rim is worthwhile and the cost of rebuilding isn't too high (it might be if you pay a shop to do it.)