Pressure drop through the heater coil and the transfer line is very sensitive to coke laydown as that reduces the inside diameter for transporting the fluid. Pressure drop is proportional to the ratio of the diameters of the pipe to the fifth power. As an example, a reduction in the inside diameter of the heater coil to 80% (coke laydown of 10% of the tube diameter around the circumference (20% reduction in diameter overall) will result in the pressure drop increasing by a factor of three.
Do you know if the pressure drop is in the heater transfer line (from the heater outlet flange to the switch valve) rather than in the heater coil? Have you done a thermal scan of the heater coils after the on-line spall? Most spalls are evaluated by the decrease in the tube metal temperature. Tube metal temperatures should be able to return to initial start of run temperatures, unless the foulant is inorganic salts or clays that may be more difficult to spall.
The two ideas that come to mind are that 1) not all the coke is being removed from the heater coil and only the areas of the tube metal temperature indicators is being spalled enough to lower the readings; 2) the transfer line has had significant coke laydown between the heater and the switch valve and that has resulted in higher heater inlet pressures. Infrared thermography will determine if all the coke has been removed during the on-line spalling procedure. A pressure survey is needed to determine if the transfer line is fouling.