It depends on how you are using it. I'm assuming you are talking about the lansky guided system.
The bevel angle will be smaller the closer you are to the pivot of the guide rod. If you clamp the blade near the handle, generally the bevel will gradually become taller towards the tip of the knife because the distance from the pivot will normally increase. You might be concentrating too much on one area/wearing away unevenly, but that would be a guess.
Is not about the presure, the strokes, not-knowing how to use it or that the system is wrong... is just pure geometry. That's it. If you want an even bevel you need to have the same distance between the pivot point and the contact patch of the sharpening stone on the blade through the WHOLE LENGTH OF THE BLADE. And you don't have that because the further from the clamping point you get, the longer the distance gets, therefore the angle lowers and the edge becomes more acute.
This
There is no way arround it, you will have to live with that. If you can't, you will need a system with a pivot point that slides on a fixed height guide paralel to the blade to insure the stroke is always perfectly perpendicular to the edge and from the same distance to the edge.
And that will only work with perfectly straight edges (or portion of edges) once you reach a curve or the upswept of the tip, all this precission goes out the window.
And this:
Mikel
I used to think the same way about guided sharpening system with a pivot, because I too have the issue of uneven bevel from flat part of blade towards the tip. I kept on thinking maybe it was the distance from the pivot that may have caused that. But eventually, I released it was only half the truth. It's hard to explain, but I did see this video which could explain why the sharpening angle remains the same on the flat part of a long blade, so as long the stone could reach it:
Please do feel free to correct me if wrong, I think the reason why 'uneven' bevel from flat to tip of a blade could happen, is not because the stone is further away from pivot, but it's all about blade geometry(since not all edge is just a straight line) and whether we could clamp at strategic location to compensate/compromise the situation.
Because of the primary grind/shape/stock thickness of the blade, the BTE thickness of the blade could be different if you trace it from the ricasso to the tip of the blade. And with the different thickness BTE, even sharpening at the same 'angle', the bevel will certainly look different.
This phenomenon may not be very prominent, if the tip don't have a huge upswept and the blade has relative thin stock.(For example, Spyderco knives). But if the stock is thicker and it's a straight back, then it maybe become more noticeable.
Also, if a blade is 'tall' in height, and has a straight back design where the tip is swept up high, then the sharpening angle of the flat edge vs tip could again be slightly different. To exaggerate this, imagine a knife with a height of 12" tall blade and has a straight back design--the angle of the stone making contact with the flat edge vs the tip will for sure be different.
In reality, I think it's always a combination of the factors above and whether we could strategically clamp at an optimal position to mitigate those factors.
Instead of always clamping vertically to the spine, one could choose to clamp at an angle to offset the uneven bevel and that's why the marker is important tool to tell us whether the current clamp location is indeed a good compromise.(Not saying an 'optimal' clamp position could solve everything, but it surely would help reduce the bevel difference to some extent.)
I'm sure this is nothing new or even basic to experienced user, but it was an a-ha moment to me when I first realized it. In practice, many may not care to have an absolute even bevel from flat to tip, so YMMV.