I don't think it was your feeble brain. I think it was jump the shark. An earlier poster said Heinlein got loony. As a certified Heinlein fan, I have to say I agree. He really started recycling the same stuff. This criticism does not detract from the wonderful body of work he did create.
I must admit I really enjoy about 75% of Number of the Beast (up until they meet Lazarus Long). It's not good Heinlein, but it's very entertaining in a fanwanky kind of way.