The nut above may be tight, but it does not fully engage the threads on the bolt. Good practice is to always leave at least 1/2 to 1 full bolt diameter protruding from the nut. Like this:
Washers need attention, too! A washer is a device to distribute the load from the nut evenly across the bolted surface. If the surface is flat and the backing hole is just barely larger than the bolt coming through it, then a non-hardened washer may do, but if it is subject to bending or is suspending a load overhead, then the washer should be graded the same as the bolt and nut (i.e. SAE Grade 5, or 8, or ASTM 305, as determined by the engineer certifying the system).