The second player presses and nothing happens
Four checks in order for when the second player presses keys and the character refuses to move.
The scene repeats in a lot of homes: the first player is running around, the second player presses everything and the character stands still, and after thirty seconds both people are talking louder than the situation deserves. In almost every case the keyboard is fine and the game is fine. Something smaller is eating the keys, and it is always one of the four things below. The list below is ordered by how often each one turns out to be the cause.
Start by clicking once in the middle of the game frame. The keyboard only counts while that window holds focus, and an embedded game frame is a separate window as far as the browser is concerned. If you clicked outside a moment ago to answer a message, focus has moved, and every key after that is being typed into nothing at all. Press Escape once as well, since some frames keep a menu open behind the view.
Next, check the Windows Sticky Keys setting. Pressing Shift five times in a row brings up a prompt asking whether you want Sticky Keys, and if somebody once said yes, the system begins merging consecutive presses into combinations. Inside a game the symptom is easy to spot: the first key registers, the second is swallowed, and the third works again. The fix is to open the accessibility settings, turn the feature off, and untick the shortcut that turns it back on.
Third, confirm that the game actually uses the arrow keys. Plenty of two-player games accept W A S D for both players, or give the first player the arrows and the second player the I J K L cluster. Without reading the key line under the cover, the person on the right presses correct keys for the wrong game, and nothing on screen tells them so. Scroll to the key line, read it aloud, and hand the correct half to the correct person.
Fourth, look at the number pad. On many compact keyboards the arrow cluster shares its position with the numeric pad, and while Num Lock is on those keys send digit codes instead of arrow codes. The game receives meaningless numbers and ignores them. Turning Num Lock off puts the arrow cluster back to the job its labels describe. Many compact laptops print both sets of labels on the same keycaps, in two different colours, which makes the confusion worse.
If all four checks pass and the keys still do nothing, open a fresh browser tab. Extensions that block keys, translate pages or draw virtual keyboards intercept keyboard events before the page ever sees them. Loading the site in a private window is the fastest test: if the keys work there, the culprit is in your extension list. Go through the list one by one, disabling rather than removing, and reload the page after each change.
It is also worth checking which keyboard layout the operating system is using. A machine switched to a French or German layout swaps A with Q and Z with W. The player presses W, the machine sends Z, the game does not recognise it, and the character stays put. A wrong layout also breaks the comma and full stop keys, so if those two feel odd as well, layout is almost certainly the answer.
One cause people rarely consider: the browser page is zoomed in or out. At higher zoom levels some game frames get cropped at the bottom, where the start button lives, so the first click lands outside the frame entirely. Press the shortcut that resets zoom to one hundred percent, then click the middle of the frame again, and the problem usually disappears in seconds. This one is easy to miss because the page still looks normal, only the game frame is cropped.
Finally, swap seats. If the second player presses the first player keys and the character still does not move, the problem is the game rather than the hardware. If swapping seats makes it work, the problem is that half of the keyboard, and at that point plugging in a second USB keyboard settles it for good. That test costs thirty seconds and tells you whether to change the game or change the keyboard, which are two very different afternoons.