Most 'straight-edge' blades, such as sheepsfoot blades and others, will have some lateral warp or bend to them, straight from manufacture. If you hold the blade at eye level with the edge up and the tip pointing away, you can see the lateral curvature in one direction or the other, like you'd see the warp in a length of 2x4 lumber, looking along it's narrow edge. OR, if you lay the blade flat on the stone and look into the cutting edge from eye level, you can see the difference as you flip the blade to each side on the stone. When sharpening blades with this sort of warp on a FLAT stone, only the tip and rear of the cutting edge will be in contact on the stone when sharpening one side of the blade, and only the center portion of the cutting edge will be in contact when sharpening the other side. With only the tip and rear of the cutting edge in contact from the one side, those portions of the edge will be worn away more rapidly during sharpening, so the edge profile starts to curve, and the previously-sharp tip will begin to round somewhat. It only takes the tiniest bit of lateral curvature or warp in the blade to affect sharpening this way. If you do a lot of leather or fabric strop-sharpening of these blades with aggressive polishing compounds, the effect will likely be even more extreme, rounding off the tip especially, due to the strop material conforming around the tip.
If the blade is thin & flexible enough, sometimes finger pressure can be applied to 'flatten' the warp somewhat on the stone, so the full length of the cutting edge is in contact. But that's difficult and very fatiguing to do with full control and the results still won't likely be as perfectly straight as you want them to be.
You can also use a stone narrow enough to focus only on short segments of the cutting edge at a time, so you're not needlessly grinding away the tip and the rear of the edge when you're only wanting to tune up the central portion. Another way to do that is to round off or 'radius' one edge of your flat stone, so the tip & rear of the edge can be better isolated on the stone while sharpening. For me personally, isolating the tip for the work it actually needs is the best way to keep it in shape with a minimum of rounding off.
As a last resort, if the curvature of the edge profile gets too far out of hand, you can grind the edge profile completely flat again, with the blade vertical (spine straight UP) to the stone, as if trying to cut through the stone. Once straight again, new edge bevels would have to be reset & the edge sharpened again. But that's a LOT of work and the same dynamics caused by the blade's lateral warp would still be in play while trying to reset the edge.
(joking)
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