Can't get Select, Modify, Contract to work on a layer

I'm editing a Photoshop graphic as i need to contract a layer. When I first did this graphic I had no problem selecting, modifying and contracting the layers. Now a few days later I have opened the file and when I select a layer (specifically the Cut Area) and select, modify, contract....the marching ants do contract but then I can't apply that contraction and it just reverts back to the original size.

I'm fairly new to Photoshop so I'm hoping it's just something straightforward that I'm not doing!

TIA