Potential problem is
1) Insufficient AC isolation of the DC bias network.
2) Improper grounding of the amplifier.
3) Type of capacitor used.If the operating frequency is at 12GHz and you are using normal DC block capacitor, the ESR will be high leading to gain drop. (Recommend: ATC 600S series capacitor)
4) Substrate used. Again if the operating frequency is at 12 GHz, using FR4 will lead to significant gain drop.
If you want to know more, tell me your operating frequency, substrate used, and type of DC block capacitor being used.