The pitch is fixed, so the only question is which hole the pin is in
Three things about a belt, and the whole page follows. It has five holes at a standard pitch of twenty-five millimetres — an inch — with the tail end skived thin and sealed, and the buckle end clamped inside the buckle head by stitching or a rivet. That pitch is not arbitrary: two holes in front of the pin and two behind is the allowance for a heavy meal, a thicker pair of trousers, or a hot day.
So the pin tells you the diagnosis directly. In the middle three holes, the belt fits and you should not touch it. In the first hole — the tightest — you have used up the allowance and it is still tight: it is too short, and no amount of punching will lengthen it. In the last hole and still loose, you have used the allowance the other way and the answer is to shorten it. Which is why "I'll just punch one more hole" fails: you cannot add a hole at the buckle end, only at the tail end, so a new hole makes the tail hang longer and does not bring the pin back to the middle.
And there is a second reason, which is about where the leather bends. Every time you tighten it, the strap folds at the buckle. Put the pin in a hole further out and the fold moves, but the old fold stays — and leather that has been folded thousands of times in one place has lost its structure there. That is the crack you can see across the strap at the level of the old hole. Shortening the belt is the only repair that moves the fold back to sound leather and re-centres the pin at the same time.
Three faults, three answers
Pin in the end hole is a length problem, and it is answered at the buckle end. Cut the stitches or drill the rivet out, measure the amount to remove as a multiple of the pitch rather than by eye, cut square with a try square, skive the cut edge thinner so it does not bulge inside the head, and refit with waxed thread in a saddle stitch or a new rivet and washer. Never cut the tail: it is skived and sealed, and a cut there leaves a raw edge that frays from day one.
A hole stretched into a slot was made with an awl, a nail or a drill. Leather is a network of fibres: a punch cuts a clean circle and the fibres close around it, while an awl pushes fibres apart, so the hole closes when the awl is withdrawn and what is left is a slit held open by stretched fibre — which then tears. Enlarge it properly with a punch of the right size (about half a millimetre over the pin) and chamfer the edges with a burnisher or fine abrasive, because a sharp hole edge wears the pin.
The buckle rocks on the strap is the head fixing: stitches have rotted or a rivet has stretched. Open it, look at what is there, and redo it — saddle stitch with waxed linen through the original holes, or a copper rivet with a washer set with a hammer. Do not simply tighten the screw if it is a plate buckle, because a screw that has come loose once will come loose again unless the leather behind it is sound.