Michael was frantic to get Fredo in the car with him and on the plane out--probably because he wanted to pump him for info on who else was involved in the plot against him. And, as soon as he's alone with Tom in Vegas after escaping from Cuba, he asks, "Where's my brother?" But when Tom replies that he's probably in NY, Michael doesn't seem frantic. "Tell him I know Roth mislead him..." etc. He could have had Tom arrange to have Fredo snatched off a NY street and brought to him almost immediately, or even have him whacked in the street. Instead Michael waited weeks, or even months, until he got caught in the perjury trap.

I've never understood Michael's sudden loss of urgency in getting Fredo to the compound.