Did you check your PowerBook's DC adapter? Does it look like this?
If the DC voltage is too low from the power brick, then the battery pack won't charge fully. It also takes a VERY long time for the pack to first charge with fresh cells. I think the pack I built took 3-4 hours to charge fully. Also MAKE SURE you unplug the power brick from the laptop when you want to run off the battery, because if you just unplug the power brick from the wall and leave it connected to the laptop, the laptop will back feed the power brick and it will drag the batteries down quick.
Make sure you're getting 7.5v out of the power brick. If you're not, there's a pot inside the power brick to adjust the voltage output. Also make sure that you have recapped your power brick, because they have a bad tenancy to have leaking caps that cause havoc on the output. There's one pencil thin capacitor in particular that I don't remember the value of, but it spews its guts everywhere and makes a huge mess.
Besides those issues, I'm not sure why your pack has such a short life. If you used the same cells as I did, and are using it in the older B&W PowerBooks with a BlueSCSI or SCSI2SD, you should get a really long battery life. There's always the possibility you got a couple of bad cells, you could open the pack and check if any of them are low.