The problem with STC (Sensitivity time controlled) based on pin diode.

[Dines]

I'm using STC (Sensitivity time controlled) pin diode for our applications. Attached the schematic diagram below for the reference.
I tried to measure the attenuator range by passing RF signals at input but I noticed 20 dB to 25 dB attenuation happened without applying an external bias current.
In theory, it should not attenuate . It is constantly attenuating at 20 dB to 25 dB even if we apply the external supply current.
What is wrong with this circuit? Can anyone please share your thoughts on this?

 

[G4BCH]

How are you isolating the control current feed line from the signal path, and are you sure that the diode current is zero or that they are reverse biased.