Paula,
I have had some working moms have steadily decreasing supplies of pumped milk.
 Some things to check:  are any of the pump parts worn? is mom using the most
effective pump (she might do better with another model or another brand)?  Is
her life incredibly hectic?  (We all know the answer to that one!)  I usually
check out the pump, offer to switch it, replace the valves in the kit, and
offer moral support.  Some moms find that stimulating their nipple by gently
pinching the base between 2 fingers increases the MER and allows them to pump
more.  As baby gets older, she can be given solids and diluted juice at the
sitter;s and mom can nurse when she is available.  I encourage moms who are
having pumping problems to make laying around nursing baby on the weekend a
priority.  This often helps more than anything else.  In general, I find some
moms are good pumpers, they can pump for 2 years with no decrease in supply,
others are not, they are not adequately stimulated by the pump.  The natural
focus on the amount of milk that is coming out (which many working moms
describe as their obsession) can also be inhibitory...
Catherine Watson Genna, IBCLC