Every puzzle has a wall.
You have the border, the buildings, the interesting bits. What is left is four hundred pieces of near identical sky and a growing suspicion that some are missing. Nothing has fit for twenty minutes.
Nine things that get you through it. In order of how much they help.
1. Sort by shape, not colour. This is the whole answer for skies and water. Split your remaining pieces into groups by their nubs and holes. Two out and two in. One out, three in. Three out, one in. Four groups covers most of them. Now, when you have a gap that needs two out and two in, you are choosing from ninety pieces instead of four hundred.
Do this once and you will do it forever.
2. Work the gap, not the pile. Amateurs pick up a piece and look for where it goes. Faster puzzlers look at an empty space, read exactly what shape and colour it needs, then go find that. Solve the hole, not the piece.
3. Turn the light around. Move your lamp so it rakes across the table at a low angle rather than shining straight down. Suddenly the pieces have visible edges and subtle differences in tone show up. This one feels like cheating.
4. Rotate the board. Give the whole puzzle a quarter turn. Your eyes have learned the picture in one orientation and stopped really looking. New angle, new information. Genuinely effective and slightly ridiculous.
5. Use the box, properly. Not just for the picture. Hold the box lid next to your gap and look at the actual texture and gradient in that spot. Most people glance at the image once and never look again.
6. Walk away for ten minutes. Put the kettle on. Hang the washing. Come back and the piece is obvious. Nobody has a satisfying explanation for this and everybody who puzzles relies on it.
7. Get a second pair of eyes. Hand the problem to whoever is in the house. They have not spent an hour staring past that piece, so they see it. This is also the moment a solo puzzle turns into a social one.
8. Check the shape twice before you force it. If a piece needs pressure, it is wrong. A good puzzle clicks. Forcing one piece can cost you an hour later, hunting for a fault in a section you thought was solved.
9. Accept the last piece problem. Sometimes a piece is under the table, in the box liner, or inside a cushion. It is almost never actually missing. Check the floor before you write to the manufacturer.
One more thought, and it is the real point.
The wall is not a failure of the puzzle or of you. It is the part of the activity that is doing the most for you. That is your attention held steady on a small, difficult, low stakes problem, which is close to a definition of rest for a modern brain.
Research on puzzle play has measured lower stress hormones and improved sustained attention afterwards (Frontiers in Psychology). The hard part is where that happens. The easy part is just the warm up.
So sort by shape, move the lamp, and enjoy the sky. It goes in eventually. They always do.