Pets have unique requirements for line of sight. On top of the standard rules every class
deals with (you must have line of sight to target and hit stuff), Summoners have two more:
If there's a wall or obstruction between you and your pet,
you won't be able to use PAs or normals for a few seconds until your pet returns to you.
It's easy to tell when this happens because if you try to press a button, neither your character nor your pet
will respond. Unfortunately, this includes your pets' recall PAs - so your only options are to
restore line of sight to your pet, wait, or switch pets.
If there's a wall or obstruction between your pet and your target,
you may use PAs or normals - but if this action involves moving the pet (normal attacking an enemy with a Melee pet,
using Synchro Zan), your pet will not move. Thankfully, since you can use PAs you'll be
able to recall your pet.
Sidenote, pets get stuck in the floor sometimes. This can happen when the part of the boss that your pet is locked onto
goes through the floor, or beneath the arena.
Don't fret - if your pet doesn't return on its own, you can fix this by switching pets.