Howcome when i tied a fishermen knot on my line my crankbait fell off?

just bought a crank-bait the other day and was fishing in a lil pond near me, at first i was getting good action and hard bites. so i casted out again and it fell off. normally i tie the fishermen knot cuz that's all i know how to tie because that's what i use for my hooks and seemed itd work for my crank-bait let me know if this is fine or if there is another easy knot put give me directions please