A bowl of egg whites that refuses to whip is almost always contaminated with fat. The sensitivity is extreme by the standards of most cooking, and it follows directly from how the foam is built.
The foam is a protein film around air
Whisking egg whites stretches and unfolds their proteins, which then link to one another and form a continuous film at the surface of each bubble.
That film is what holds air in place, and its strength determines whether the foam stands up or collapses into liquid.
Nothing else in the white contributes structure, so anything interfering with the proteins interferes with everything.
Fat interrupts the film
Fat molecules position themselves at the same air-water boundary the proteins are trying to occupy, competing for the surface.
Where fat sits, the protein film cannot form a continuous sheet, so the bubble wall has weak points and the air escapes.
A trace of yolk, a greasy bowl or a plastic vessel holding fat in scratches is enough to prevent the foam reaching full volume.
Acid and sugar stabilise it differently
A small amount of acid helps the proteins unfold and link at a moderate rate, producing a stronger network and reducing the risk of overbeating.
Sugar dissolves into the water phase, thickening it and slowing drainage from between the bubbles, which is why sweetened foam holds far longer than plain.
Sugar added too early competes for water and slows the proteins, which is why it is generally introduced gradually once volume is established.
Overbeating breaks it in the other direction
Continued whisking past the point of full volume forces the proteins to bond too tightly to one another and squeeze water out.
The foam then looks grainy and begins weeping liquid, and no further beating recovers it because the bonds are already formed.
This is why recipes describe a visual endpoint rather than a time, and why the endpoint arrives sooner in a powerful machine.
Copper and cream of tartar do related jobs
Copper bowls have long been used because copper ions bind to sulphur groups in the proteins and slow the over-tightening that causes graininess.
Cream of tartar achieves a broadly similar protective effect chemically, which is why it appears in recipes written for ordinary bowls.
Both widen the window between properly whipped and ruined, which matters more in a home kitchen than any gain in final volume.