Reading VCC does not change after SETUP

I am making an ultra low power Pro Mini Data Logger running on 2 AA batteries.
With my general code running about once a minute.
I want to display the battery voltage on my (Nokia) LCD screen.
I have used both "readVCC" subroutine and the VCC.h library but I find the same issue with both.
When I startup the system it displays a fairly accurate voltage agreeing with my DVM. But, when the battery voltage changes the system still reports the voltage at startup.

Test case: I have two battery packs, one strong and one weak. When I reboot on the lower battery it shows that reading and likewise for the higher battery. However, when I connect them both and disconnect them one at a time (no reboot) and voltages changes ~ 0.5 volts the Arduino voltage reading does not change.

Anyone else seen this? How can I get up to date measurements?

Thx Trebuchet


Got it.
First time through "loop" I read the VCC voltage and then I put the Arduino to sleep. Part of the sleep routine is to clear the ADEN (AD enable) bit. Since I never expressly reset it I kept reading the initial conversion over and over. I now set this bit in my readVCC routine and it works as advertised.