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.

It maybe be if you are on a server that they have a ground item clear. I have been on many that do that in like factions so check out there rules and stuff to see if they have that in place.

Here are the ways they could disappear and I got ideas from Ave:

1. They despawn. You're correct about that. I died before and retrieved my items, but not all of them. Dropped items could despawn if they aren't taken for a long time.

2. Someone else picked it up. If you're playing multiplayer, like in a server, someone else could've collected them. Try to ask whoever found a bunch of your dropped items in chat.

3. They were burnt. Did you fall in lava? Lava could destroy dropped items and you can lose them forever. Cacti can also destroy them.

4. Servers remove it. If you're on a server, they might remove any dropped items not in use.

5. You probably checked the wrong place. There are many places where you could've died. Check those areas.

6. A random bug. Though not known worldwide, sometimes the game could be bugged out and they could disappear. Chunks that are bugged too, since they can load up areas. It may also because of memory too.

Your question isn't very clear. If you have any command blocks running in your world, they might be removing items, or the items could just be despawning.