[R] stack object layer names not visible

[Fabio Berzaghi]

I use to be able to see the layer names of a RasterStack by just typing 
its name but now I only get this

class       : RasterStack
dimensions  : 70, 180, 12600  (nrow, ncol, ncell)
resolution  : 0.5, 0.5  (x, y)
extent      : -80, 10, 50, 85  (xmin, xmax, ymin, ymax)
coord. ref. : NA

If I do plot(s) then I can see all the layers and their names.

Why is this happening?