The platform may well have a sag in that area defocusing the beam.
After verifying the beam alignment across the entire platform, check the focus. You can get a crude measure by punching pinholes in paper: progressively larger holes as the laser head enters the trouble zone are a Bad Sign.
Set the controller for a manual pulse at a reasonable power and a few tens of milliseconds so the results don’t depend on your button-pushing reflexes. If you have a Ruida controller, this earlier discussion may help figure it out:
To figure out where the focus went, compare ramp tests across the platform. I made an overly complex fixture, but this is the general idea:
