Temperature on the surface also depends heavily on the thickness of the prinbed. I do have a lot of delta in the temperature too with my printbed because I got a very thick aluminium plate on top of the heated bed to straighten it out and on top of that there is glas with pei for a perfectly level finish.
Hi, the thermistor on my CTC also didn’t heat up enough, was causing issues with ABS and Sitall glass, so I replaced it, details here Coding Chase: CTC-3D and Sitall 19 Hope this helps - also, on a later post I used an infrared monitor to read the temperatures, should help too. EDIT: just to confirm that in my case, replacing the sensor did the trick, upgrading the firmware was a good plus as well.
You will need to change the thermosister for a 100 thermosister Like these on the bay, item number 191885437601 That should fix your problem. The more layers you have on top of the bed the more you will need to increase the bed temp to get a proper surface temp