I've had it tell me I messed up when I watched myself click every one. Usually the second time it works, but it is definitely a glitchy system.
Trying another browser as Lisa suggest might help.
Oh, and if you see a triangle with a tip cut off, it's still a triangle. So don't be too literal about it. They are either squares, triangles or circles, there are no trapezoids. So if it looks like a trapezoid, it's a triangle.
Time to build a bigger bridge.