You can't see them because there no space available and so Tableau suggest you to not show it.
If you want to see it anyway, you can go to the Label mark of your worksheet, and check the flag "allow label to overlap".
Remember that selecting a specifc slice you can move the label, but I would not reccomend it if your chart may change due to filters.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…