Depending upon the radioactive isotope used, eventually all OA2 tubes will fail when they cannot ionize to start up. A few years ago I ran across a reference regarding this with all tubes that have a miniscule amount of radioactive materials inside of them. Some isotopes have already gone through several "half-lives" and may be approaching that point in time where they will not strike up and act like regulators.
The element and amount varies by tube type and manufacturer. For a more definitive reference;http://www.tubecollector.org/documents/radioactive.htm