Risa

Who says he didn't? We only know he sensed that Luke was strong in the force during the death star trench run because he said it out loud. Presumably he has encountered many people with relatively strong force potential, but that's probably something he's going to keep to himself unless he has a reason to act on it. She's a prisoner that he's unlikely to recruit, and it's not like she's had any training. Hell, he probably dismisses people like her as being too old to start training anyway. (And if we bring in stuff from outside the movies where they explain that force sensitive people often become leaders, or larger than life figures that shape the course of events, then one would expect a lot of those people to run into Vader eventually.)

It may also be that force ability is more apparent once someone actually starts using the force. That would be logical and consistent with the fact that Luke is only really noticed by Vader once Obi Wan started telling him to use the force, and the fact that Vader and Luke could sense each other in space over Endor, but apparently that didn't apply to Leia.

And again, he wasn't trying to reach Luke on the radio to make him an offer once he noticed the force was strong with him, he was trying to kill him. The empire wasn't determined to capture Luke until the emperor was able to sense him making waves in the force, when he was at the point where he was already using a lightsaber and capable of telekinesis, and when he had been identified as Vader's son. At that point he had demonstrated actual ability, not just wasted potential, and the familial connection offered a plausible way to work around his opposition to the empire.