80 grit is way too coarse a grit to gain a really spherical surface. You can get close by a good short thick wet of grit but there will always be some non-spherical. I'm usually happy when the 4" spherometer that I use is within .0002" over the surface.
It's when you get to the finer grits that the surface becomes a lot more spherical. I'll also note that insufficient grit will also cause the center to go deep as the edges aren't being supported by the grit.
Go to 120 to 220 and get rid of the 80 grit pits and proceed on with the following grits.
Also make sure that you don't go chasing pits around the mirror as there will always be one or more deep pits made with any particular grit.