Friends who open your Wishpicks list can reserve a wish so nobody else buys it, and by default your list doesn't show you what's reserved.
People with the link, your followers and anyone you invited as a Viewer can reserve as many wishes as they like. You and anyone with Full Access or Contributor access can't: you mark wishes as fulfilled instead (see "How do I mark a wish as received?").
While reservations are hidden, your list looks the same whether nothing is reserved or everything is: no chips, no faded photos, nothing inside the wish. That's how reservations keep the surprise. To look anyway, see "Can I see what's reserved on my list?". Want an email when something's reserved? See "How do I choose which emails I get?".
Can two people reserve the same gift?
No. Each wish takes one reservation, and there's no quantity. If two friends reserve it at the same moment, the second sees "Someone just took this". It only covers Wishpicks, though: someone who never saw your list can still buy the same thing.
Why can't my friends reserve?
Usually because the list only takes reservations from signed-in people, reserving is off, or they help run the list.
Only signed-in people can reserve: they sign in first, or you switch Who can reserve to Anyone.
Reserving is off, as on some older lists: friends see "[your name] has turned off reserving on this list". Picking either Who can reserve option turns it back on.
They help run the list with Full Access or as a Contributor.
It's set to Only me and they haven't joined (see "Who can see my wishlist?").
Only one wish won't take it? Someone's probably reserved it already, which your list hides by default, or it's marked fulfilled.
"Can I see what's reserved on my list?" shows where to change Who can reserve.
Can I cancel someone's reservation?
Yes, while reservations are showing on the list, and the gifter isn't told. People with Full Access can too, but Contributors can't, and a wish marked fulfilled has no Cancel reservation.
If reservations are hidden, show them (see "Can I see what's reserved on my list?").
Tap Reserved above your wishes and open the one you want to free up.
Tap Cancel reservation, under "Someone reserved this." or the gifter's name.
Confirm with Cancel reservation. You'll see "[wish] is free to reserve again".
Reserved months ago and never bought? The gifter may have reserved without an account and lost that browser, so they can't cancel it, but you can. Your own reservation on a friend's list? See "How do I cancel my reservation?".
Do reservations expire?
No. A reservation lasts until someone cancels it, or the wish or list is deleted.
Tips
When you send the link, ask friends to tap Reserve on what they pick, so the next person sees it's taken.
Reusing a list next year? Last year's reservations are still on it, so free up any that never turned into gifts.
Before freeing an old reservation, check the wish for a name or message and ask that person first: they may have bought it already.