The younger birds might not be eating the OS, is it in a separate container easily accessed?
I have sprinkled some out in the run and just a few pieces on top of feed to get them familiar with it....have also heard of sprinkling just a touch of scratch grains on top of OS in separate feeder.
The younger girls just starting to lay may be also be stressed, and pushed around by the older birds keeping them off nests, feed and OS container. Sometimes it's better to let things progress until they find their groove, more changes can also cause stress.