It could be, like Mike said, that your wedge is too soft. Maybe Pietta is cutting a corner here and there as of late. If a wedge, either old or new, has been damaged or deformed from whatever cause. I would replace it. Replacement wedges (made by both Pietta and Uberti) are not absolutely consistent. And since very little difference in the dimensions of wedges makes a big difference, then a new wedge might be a good fit or it might not be. In any case, a replacement wedge will most probably not be too narrow. That is unless the problem is one of the other possible issues that Mike mentioned. I hope not.
On the subject of wedges, if you or anyone is interested, my experience with Pietta's open-top revolvers out of the box has been that the wedges are slightly too wide. They are not adjusted at the factory for a perfect fit. So if they can not be tapped into place at that point, they are forcibly hammered in, sometimes even to the point of causing a visible deformation at the rearward edge of the wedge slot on the right side of the barrel. A wedge is very difficult to remove when driven in so overly tight. It has to be hammered out. I use a thick dowel or other piece of wood between the hammer and the wedge after placing the revolver on a mattress or stiff cushion to ensure against damage.
A wedge that is too wide needs to be narrowed for a proper fit.
That is a pretty simple and easy process. I do it by removing just a bit of metal along one side by running that edge back and forth lengthwise over a piece of 800-grit wet-or-dry sandpaper that is placed on a hard, flat surface such a counter top. The wedge should be held firmly when doing this, keeping the entire edge being sanded perfectly flat against the sandpaper and maintaining consistent, even pressure. This is to preserve the angle and flatness of the edge surface and the 90-degree angles with the top and bottom of the wedge.
The sanding is done little by little with frequent visible checks of the edge and frequent checks of fitting into the gun until the wedge goes in as it should. Generally speaking, that would be fitting in with firm finger pressure until the hook of the spring clears the wedge slot on the other side of the barrel. After narrowing a wedge, the adjusted side can easily be cold blued.
And from which side should the width be reduced? I read somewhere on this or some other forum that the side to be sanded when narrowing a wedge should be the side that faces the muzzle. No explanation was given, and I don't know what the necessity would be for that. Maybe somebody can explain it.
Meanwhile, since the width of a wedge beyond each side of the spring is commonly not the same, I have sometimes chosen to narrow from whichever edge is further from the spring. If both sides are about the same, I narrow from whichever side will result in having the wedge spring be most directly positioned under the wedge screw. In some cases, I have narrowed a little from both sides to preserve a good alignment of the spring and screw. In other cases, I decided to narrow from the edge where there was some kind of minor irregularity or damage.