The same thing happened to me with my milk supply after I started back
to work full-time, 12-hour night shifts.  I tried fenugreek, which
worked a little, for a couple of weeks, then the supply decreased
again.  I ended up using tincture of goat's rue, which worked
immediately to double what I was able to pump, and kept working as long
as I took it.  After a while I was only using it during the times I
worked and "power-nursed" the rest of the time, wearing the baby in a
sling and sleeping with her at night. I eventually stopped using it
altogether when she started sleeping through the night and I stopped
pumping at work. I believe if you look in the archives under "goat's
rue", you will find my previous post about what it is, where to get it
and how to use it.  It worked really well for me, and my daughter never
had a single drop of ABM, I'm happy to say.
Katy Giller
Boulder, CO