It is a good practice to do before and after (on line spalling) thermal scans of the new heater, to learn how it behaves.
Did you do a scan right after the unit startup? This is a good reference to compare.
Too much flushing and/or steam will affect the pressure drop through the heater coil.
Flushing can also come from heater charge pump seal (if you have a Plan 32).
In a coker I worked we had pressure transmitters without flushing, that stopped working after a while.
We also had about 1.5 inches of coke formation inside the transfer line, but that took a few years.
Hope this helps