When the player teleports, e.g. using a Nether Portal, end portal, or the /tp command, any pets following them will sometimes fail to teleport before the chunk they are in is unloaded. At that point, the only way to retrieve them is to return to the place they were lost and try again.

1. Get pets following you
2. Teleport far enough that the pet is now out of chunkloading range.
3. The pet may or may not teleport to you.