Diagnosis
This is almost always a physical alignment issue, not a lens problem. The projector isn't perfectly square or level to your screen or wall.
How to Fix It
You need to fine-tune the physical positioning. 1) Turn on the test pattern in the settings (usually a grid). 2) Loosen the adjustable feet on the bottom of the projector. 3) Get down to eye level with the projector and slowly adjust the feet, focusing on getting the top edge of the projected image perfectly level first. 4) Once level, scoot the entire projector forward or backward a quarter-inch at a time until all four corners look equally sharp. The PL1 is super sensitive to distance because it's ultra short throw. A millimeter can make a difference.
If That Didn't Work
If one specific corner stays stubbornly blurry, your wall or screen surface might not be perfectly flat in that spot. Shine a flashlight across it at an angle to check for bumps or dips. You might need to use digital corner adjustment (keystone) as a last resort, but it degrades picture quality slightly.
Heads Up
Avoid using the digital focus or keystone sliders as your primary fix. Get it physically right first. Using digital correction chips away at the pristine 4K resolution you paid for.