This was, IMO, one of the problems with both the third and fourth invasions. Unfortunately, it was a problem in all directions—some hooks were too obvious, others were too subtle, some players were too dense, others too canny.
That's a classic GM dilemma. A good GM can improvise and adjust his plot hooks if things are looking too hard or too easy, but doing this in a game with the scale of BM is probably a bit different than doing it when your party is five or six people whose abilities you are familiar with and whose actions you are aware of at all times...
