The title asks it all: why can't I add a sample sentence for this character?

Perhaps because the segmenter is trying to read it as "自助" + "餐"? The sentences you add–do they show up somewhere in the list for "自助"? You could point us at a sentence you added and we could manually adjust the segmentation (you can do this yourself during sentence creation, but not afterward). Or is there some other error going on?

Thanks Nick! Problem solved!