What do you determine to be the weak point of the dwarves plan in The Hobbit?
By this I take it you mean their overall plan to travel to the Lonely Mountain, slay Smaug, and recover their ancient home and the treasure therein? Instead of specific scenes along the way?
There are a couple of flaws in their plans. The first was their lack of a plan beyond traveling there. They lack detailed maps or knowledge of the route, and rely on Gandalf's help getting there. None of them are warriors and we are not shown much evidence to believe they are capable fighters. They run away from fights whenever they can, and send Bilbo to scout danger ahead whenever they can. They are apparently unarmed until they find the troll's hoard. All this - and they hope to slay Smaug somehow! This is perhaps the greatest flaw in their plan.