I have a 8-10 year old Tormek (approx) with the original wheel. I never let it sit in the water after using it. Lately, I seem to be getting a dip in the middle of the stone that develops after sharpening just one or two HSS bowl gouges - sharpening, not re-shaping. Reconditioning the wheel between gouges helps some, but I feel like the stone has become "soft". Could this happen after a number of years? If I replace the wheel, which replacement do you recommend for HSS gouges?
It could be just a slight inconsistency in the density of the stone at one spot. I've seen it before, and they're rarely as much as 1 mm thick, usually less than .5 mm. True the wheel, rather than grading it, and see if it doesn't go away after a few more uses.
If you decide to get a new wheel, the SB-250 "Blackstone" is the best choice for HSS turning tools.