Why dosn't a new battery hold the charge

i have taken my ipod touch to a authorized apple/mac

repair shop....he has had it for going on 3 weeks, when i stopped by to check on my pampered ipod touch 2G

HE SAYS the new battery will not hold a charge, thats the reason i took it to him to replace the battery, thinking its new battery time, i am about to go get it and follow one of your repair guides, when i check on him he needed more time to solder in the battery, That struct me as odd.

any comments would help me dearly

Hugh