I like what old4570 said (the guy knows his batteries man!)
Were the batteries fully charged :
Did you match the batteries for current draw :
Did you check to make sure the batteries were not faulty in any shape or form .
Multi cell operation requires care and knowledge , especially for the batteries .
If you failed to use due care , then this is operator failure .
1/ Make sure the batteries are all charged to the same state [ voltage ]
2/ Rest the batteries after charging for at least 1 hour and test voltage state ..
3/ All cell should match
4/ Test current discharge in another light , they should all be close
5/ When using a multi batt light , often test batt state to check for even discharge
6/ Buying cheap cells means you may need to take more care matching cells
7/ failure to use due care , failure to educate yourself to the correct use , may result in possible problems , resulting in possibly anything but the operator taking the blame .