First you haven't shown us your DbContext and your configuration, so it's pretty much impossible to tell you where you went wrong. But, by what you have posted, i think you're over-complicating things.
Next, all of these types will end up in the same table, Orders. Your discriminator for Lab and Radiology reports is redundant.
public abstract class Order
public class LabOrder : Order
public class MedicationOrder : Order
public class ImmunizationOrder : Order
DbContext:
public class MyDbContext : DbContext
{
public DbSet<Order> Orders { get; set; }
}
That's it! Nothing else needs to be done for a basic table-per-hierarchy inheritance graph.
If this is your first foray into Entity Framework, I HIGHLY suggest you pickup a book or two on it before you dive in. You're not going to learn all of the in's and out's and conventions that EF uses from a bunch of forum posts.