The Lounge Forum is not for technical help, please provide the name of the program you are using so your message may be moved to the correct program forum... A program would be Photoshop or Dreamweaver or Muse or Premiere Pro or ???
Its Adobe Photoshop and sorry Ive never used the forums before
Assuming you mean onion skin in Select and Mask........
1. Do you have the opacity/transparency slider set at greater than 0% (around 50% works well for many images)
2. Are you working on a duplicate layer with the original below? If so, the transparency of onion skin will show the layer below and therefore look as if it is not working