Yes, it probably needs you to input TAS now as a starting value, i think it was ATAG_Keller who confirmed this in another post. And i say starting value because in reality, what matters in bombing is GS (ground speed, or how fast your are actually going compared to the ground).
I usually do it the old fashioned way a la IL2: just set the altitude, start from the IAS or TAS value and engage automation. Since your actual ground speed is higher than IAS and closer to TAS (but not always equal, eg, it could be lower than TAS in case of a headwind), you will see that the sight is drifting above/below the target (it can't compensate because it "thinks" the plane is going slower/faster than it really is)
So, i simply increase/decrease bombsight speed in small steps until the crosshairs stops drifting, recenter it on the target and wait. Essentially you are using the sight's tracking mechanism to "discover" you ground speed
