I had similar issues tho on a custom designed printer. The issue ultimately was the adhesive for the silicone heater for the bed was failing ans slowly delaminating from the bed. When I felt the heater it was plenty warm… But when I felt the center of the bed where it prints it was a lot cooler. I had to replace the whole heat pad. I don’t know what yours uses to heat the bed but my guess would be that it is also on the way out.