The tubers of this species tend to be small (about 1/4" or less) and usually
have an orange/brown colour - which makes them easy to miss.  I would hang
on to all the soil in case they do come up again.  I assume that you followed
the summer dry dormacy requirements (otherwise they may have rotted, though
this plant is hardy and tolerates "neglect").
BFN
+-----------------------------------------------------------------------------+
|    John Taylor  [Catweasel]    |   Royal Melbourne Institute of Technology  |
|  rphjt@minyos.xx.rmit.edu.au   |        Melbourne, Victoria, AUSTRALIA      |
+-----------------------------------------------------------------------------+