Parallel buses on a breadboard
I would agree on the likelihood of a wiring error (or else a design error), but would dispute the best solution for a complex system, particularly with parallel data paths (though we don't see as many of those today).  

The problem with the preformed kits is that they restrict your color choice, and probably aren't long enough to go between multiple modules.  (I probably could be talked into paying for a set of perfectly formed ones for ground straps and very short jumpers though)

Instead, for a complex project get yourself some good 22 gauge hookup wire in multiple colors, or if you can find some that fits well, surplus solid core telecom cable to split into individual wires.

I did enough breadboarding in childhood, that by the time I was assigned to build a fairly complex system in a course lab in engineering school, I realized I needed to do it in a way that would be durable and debuggable.


I used the same four colors in quasi-spectral order for busses.  An 8-bit bus meant using them twice, but the bit position / color assignment remained constant in each nibble.  If memory matches what I'd do today, the bus colors were never used for anything else - power, ground, config strappings and selects used something else.
No wires over chips (possible exception for a rare direct "staple") 
Runs were grouped together, and often held down with short pieces of wire stuck in power buses or unused holes.


One could try to make a theoretical argument that running signals together in routing channels / bundles increases crosstalk, but the reality is that for the types of digital systems properly built on breadboards, crosstalk is not the problem.

Wires falling out, and not being able to figure out which hole they go back into, or which wire is which, is the problem.