If the batteries are getting old, they may be sulphated. Discharging a battery causes lead sulphate to build up. If it has been discharged too many times, you will have less lead on the electrodes, resulting in less capacity. You may still get it charged up to 12.6 V, but it won't last very long.
Also, what Bill said about waiting 30 minutes. You may even need to wait overnight. I just had trouble with my SUV battery. It started off at 12.4 V (a little weak) after a couple of hours, but still enough to start. Overnight it dropped down to 11.9V and was barely enough to start.
One thing you can try is a desulphater. I have managed to revive a few batteries this way, but if they are too far gone, even a desulphater won't help. (I use one from Powerpulse.) Good luck!
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