LPC1768 doesn't generate RTC clock while other LPC1768 does
If RTC does not work - and it seems to be as simple as two caps and one crystal as you said - there're following options:


there's some software setting in the microcontroller which explicitly tells circuit not to run RTC (in other words - disable it);
there's some conductor or via damaged in the caps/crystal circuit, or short circuit;
the microcontroller is having problems... this seems to be the worst case.