The ancestryDNA test wasn't intended to find Y-DNA haplogroups, it's just a side effect that the promethese program uncovers, just like being able to find some medical info. That means it's going to be hit and miss.
The thing to consider is knowing your Y-DNA haplogroup at the level you'll get from this method is only going to tell you very basic deep ancestry on your paternal line, and maybe not even that. For example, if you can determine you are R1b then join the club along with a large number of Western Europeans and U.S. immigrants. If that type of information is important to you for whatever reason, then just spend the $49 and do a real Y-DNA test. That will give you a basic 12 marker haplotype along with a predicted haplogroup, and you can compare that against others with your surname. If you decide there is a need to dig deeper, then you can upgrade without having to provide a new sample.