Why was Tobi not proactive after all missing nins got recruited for Akatsuki? As far as I know, Tobi didn't come to limelight until Sasori's death (I am talking as per the chronological order of the telly series). He teamed up with Deidara after Sasori's death. He knew Nagato and his team long back but he was not proactive with the organization at the beginning of series.

Why didn't he volunteer himself? Is it because Akatsuki work in pairs and he didn't have anyone? Or was he waiting for the right moment to join with the organization?
Tobi was the brain behind the plan. But he used Nagato to carry out his plan. He exploited Nagato's hatred towards big countries. He was dead as obito for the world. So he wanted to hide and work behind the scene so that he could keep his identity hidden from all before he could collect all bijuus. Also, Land of Rain was always a secretive nation. And Akatsuki already had strong base. It gave him a perfect place to hide himself. But towards the end as more and more akatsuki members died he was forced to come to limelight.