Common issues related to a stiff thumbstick include dust or dirt getting into the DualSense (PS5) Controller V4, internal misalignment or stick drift issues. Here’s how you can fix it:
Check for Dirt & Debris
Move the thumbstick in circles and see if anything sticks.
Then shoot a can of compressed air around the base of the thumbstick.
Clean edges having a soft brush or cotton swab using isopropyl alcohol (70% or better).
Reset the Controller