The output appears to be sorting exactly as specified. It's ordered first by SeqNum and then within each group of SeqNum they are ordered by Date (some of which appear to be blank or null).

Another option is to look up some articles on using the ROW_NUMBER() operator for creating nested ordering. That operator can let you do just about any grouping/sorting you want by creating "sort" columns for as many levels or groups you need. This method can be a bit complex, but if you give us some data examples and the desired output then perhaps one of us can give you specific examples to get your desired result.