“Nothing, really.”
It is almost never nothing. It is one of three things: they do not want you to spend money, they do not want to be a bother, or they stopped believing you would notice the specific thing. All three are solvable. None of them by asking again.
Listen for the thing they mention twice.
People say the things they want in passing, then say them again, then go quiet about it, having decided it is silly. The second mention is the tell. The mug they described down to its color. The book they almost bought twice. The town they keep saying they should go back to. First mention is noise. Second mention is a want wearing a shrug.
Look at what they replace, not what they own.
Nobody buys a second of a thing they do not care about. The headphones that died and got taped back together. The pen they keep refilling. The coat from years ago that they wear every single week. Worn out beats worn once, every time.
Buy the occasion, not the object.
People who say they want nothing often mean they do not want an object, because an object can feel like a debt. Time does not. A cleared Saturday. A meal cooked badly and eaten slowly. A day already planned so they do not have to plan it. The best gift I ever placed for someone was not a thing. It was a reservation they had been meaning to make for two years.
Match the size to the person, not the day.
An oversized gift embarrasses a private person. A small one read exactly lands better than an expensive one read approximately. The point is not the price. It is the proof.
And the one I keep relearning: if you still cannot tell, it is not a gift problem. It is a noticing problem. The gift is downstream of the attention. Go back and watch them for a week before you buy anything at all.
A wrong gift is worse than no gift. A right one is just proof you were paying attention. That is the whole trick, and it is not a trick.