st: fillin not filling in -- Stata 12

[Dorothy Bridges <dbstata@gmail.com>]

My data are as follows:

municipality     year     month
muni1             1990    1
muni1             1990    2
muni1             1991    5
muni2             1991    6
muni2             1991    7

According to the documentation and to my prior experience with fillin,
"fillin municipality year month" should give me 50 observations (ten
for each municipality -- i.e., muni1 1990 1 / muni1 1990 2 / muni1
1990 3 ... muni1 1991 6 muni1 1991 7).

Unfortunately, this does not happen.  Some of the desired 50
observations appear, but many do not.

(Of course, my actual data have close to one million observations, but
the structure is the same).

The closest archive thread I can find is this one
http://www.stata.com/statalist/archive/2007-01/msg00891.html, which is
unresolved (as far as I can tell).

Thanks in advance for your thoughts.  I am very confused.
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