Why do some keyboards miss keypresses?
Because of how keys are wired — most keyboards use a matrix rather than a separate wire per key, and that arrangement has known limits when several keys are held at once.
The matrix. Keys sit at intersections of rows and columns. Pressing a key connects one row to one column, and the controller scans rapidly to detect which. This requires far fewer connections than wiring every key individually, which is why it is universal.
Ghosting. With three keys held forming three corners of a rectangle in the matrix, the electrical path can make it appear that the fourth corner is also pressed. The keyboard reports a key you did not touch.
Blocking, or jamming. To prevent ghosting, keyboards insert diodes or simply refuse to register the ambiguous combination — so instead of a phantom key, a real keypress is ignored. This is what most people experience: holding two keys and finding a third does nothing.
N-key rollover is the solution: a diode at every key intersection, so any number of simultaneous presses register correctly. It costs slightly more, which is why it appears on gaming and mechanical keyboards and frequently not on cheap membrane ones.
Why the limits vary by key group. Manufacturers arrange the matrix so that commonly combined keys — modifiers, and the cluster used for movement — fall on separate rows and columns, which is why a keyboard may handle several keys in one area and fail elsewhere.
The other causes of missed keys, which are different problems:
Debounce. A switch physically bounces on contact, so firmware ignores changes for a few milliseconds. As switches wear, this can produce missed or doubled characters.
Wireless issues — interference, low battery, or a saturated receiver.
Dirt under membrane contacts, which is the commonest cause on older cheap keyboards.
Operating system settings such as key repeat and accessibility filters.
How to test: an online rollover tester shows exactly how many simultaneous keys your keyboard reports, which identifies immediately whether it is a matrix limitation or a fault.