It depends on the weather, If it is isn't that cold then the top layer of snow melts and then re-freezes and you get an icy piste.
So generally the higher you go, the colder it is so the less likely it is to be icy. Also the busier the piste the more icy it gets, so quieter resorts will have better snow
Flumserberg has snow making machines they use to blow snow on the icy areas.