I can't find my items after I die

There might be 4 reasons of this.

Someone is picking them up. If you are playing on a server, this is much more likely. Even if you are playing solo, some mobs can pick up items.

They are being destroyed. This depends on how you die. If you fell into lava, your items will probably drop there too, after they pop off when you die, and be destroyed. Minecraft Wiki's Item Page gives examples of explosions, fire, lava, a falling anvil, and contact with cactus.

Your game client is bugged. Can you be playing on an old version, a snapshot or using mods? They might cause it.

Server Plugins might be removing them., but this only happens if you are on a server. Only thing you can do to this is not dying, or contacting admins to remove this feature.

In addition to Ave's answer:

When dying by suffocation, for example by teleporting into a solid block, will cause items to occupy a nearby empty space. This can even be much lower or much higher than where you died.

When on the "game over" screen, the world does not freeze. This means that any item timers will still be running when on that screen. I believe entities will not appear despawned on that screen, but they will despawn regardless.