We bought a couple of the Roundabout tickets last year. Nowhere on the publicity (or at the NR ticket office where we bought them) did it say that the NR issued ticket would not work in the Subway barriers.
On changing to the Subway at Partick, we asked at the Subway ticket office and they issued two further "Zonal" tickets on Subway stock. These work for multiple entry all day, there is no need to get a new one for each trip. The magstripe is the same width, so probably, as Flying Snail says, incompatible systems. Both the NR and Subway tickets were stamped with a red S, I assume to stop you acquiring further tickets on each system later in the day.
The NR issued ticket, as Tom says, is printed as an Off Peak Day Single, with "From" as issuing station and "To" as "Roundabout". It will be for this reason, I assume, that it will not open the barriers anywhere else on the NR system, as we found out when we tried to enter at Queen Street. Had they set it up as a day ticket (like the West Midlands Daytripper) then it would probably work the NR gates.
Just looked up the ticket on the website, which states:
"Tickets can be purchased from any staffed ScotRail station, from on-train ticket staff or from any SPT Travel Centre"
Clearly the uninitiated traveller is expected to know the difference between a Ticket Office and a Travel Centre. Why they can not sell one from an ordinary ticket office is beyond most of us.
I take it that the encoding incompatability also contributes to the reason why a through NR ticket is not valid for use on the Subway to transfer, between Central & Queen Street, for example.