On Wishpicks, Reserve shows only on someone else's wishlist: not on your own list, a list you help run, a collection, or a wish that's already taken.
Check these two first, because they explain most missing buttons:
It's your own list. You can't reserve your own wishes, so you see Fulfill where your friends see Reserve. They still get the button.
It's a collection, not a wishlist. Nobody can reserve from a collection, and its things show Add to wishlist instead.
Why is there no Reserve button on my own list?
Because reserving is for the people giving the gifts, and the list's own side marks wishes as fulfilled instead. That's you as the owner, and anyone you invited with Full Access or as a Contributor. A list you made without an account is yours in the browser you made it in.
To check that friends get the button, open the list the way they do:
On your list, tap Share (just the share icon on a phone), then Copy.
Open a private (incognito) window, or another browser where you aren't signed in.
Paste the link. When it asks "Who's opening this list?", tap I'm a guest.
Open a wish. Reserve is there.
A list set to Only me won't open that way. As a guest you'll also see which wishes are taken, so skip this if you'd rather keep the surprise: see "What can people I share my list with see?".
Why do I see Add to wishlist instead of Reserve?
You're looking at a collection, or at a product in Explore or search, and neither can be reserved. A collection holds things someone recommends, and it counts them as picks, not wishes. If a friend sent it for gift ideas, ask for the link to their wishlist. If it's yours, add the picks to a wishlist and share that link: see "How is a collection different from a list?".
Why is Reserve missing on one wish?
That wish is already reserved, marked fulfilled, or reserved by you.
A Reserved chip and a faded photo: someone else took it, and the wish offers "Find something else for [name]" instead.
A Fulfilled chip: the owner has it already, and the wish says "[name] already has this."
Reserved by you: it's yours, so you see Cancel reservation (see "How do I cancel my reservation?").
Where is the Reserve button?
Inside the wish on a phone, and on the wish's card when you point at it on a computer.
Phone: tap the wish to open it, and Reserve is in the bar at the bottom of the screen. It's also the first row of the card's ⋯ menu.
Computer: move the pointer over the wish's card, or click the wish and find Reserve there.
The full steps are in "How do I reserve a gift?".
What if the list shows something else instead?
Then the owner has limited who can reserve on that list, or the list takes you for its owner.
"Your list, no spoilers." at the top: someone tapped It's my list in this browser. Tap Not my list, then I'm a guest.
Sign in to reserve: the list takes reservations only from signed-in people. Sign in, and you can reserve.
"[name] has turned off reserving on this list": nobody can reserve there until the owner turns it back on. If it's your list, see "How do reservations work?".
Tips
Friends say there's no button? Ask whether they opened the wish, then check that you sent them a wishlist and not a collection.
Testing your own list in a private window? Look, don't reserve: it's a real reservation, and your list hides it from you. If you did reserve, cancel it before you close the window.
Sharing a phone or computer with the list's owner? While their account is signed in, you see the list as they do, so open the link in your own browser.