Losing power when using breadboard with Raspberry Pi 3
Breadboard is prone to unreliable contacting, so that's my first guess.

Also, yes, cheap jumper wires are relatively thin (if you can afford to do so, cut one and count the strands...).

Furthermore: motor control might involve high frequencies. Breadboard has high parasitic capacitance and inductance, it's known for bad adjacent-trace crosstalk. So, that might be breaking your system, especially if it involves MOSFETs.

I, very often, recommend that anything that reaches a more than ten pin complexity is modularized and implemented on soldered perfboard. The prototyping of the not-as-complex modules can happen on breadboard!

Point is: If you have \$N\$ contacts to be made on breadboard, and your circuit only works when all contacts are proper, then there's 1 case where your breadboard circuit works, and \$2^N-1\$ cases where it doesn't. 

Now, if all contacts have the same "properness" probability \$p\$ independently of each other, then the case that all work is \$p^N\$. 

If \$p=99\%\$ (which would be, especially for cheap breadboard, be a good value, as far as my experience goes), then your "works" probability for a circuit with \$N=10\$ contacts is only around 90.4%. For \$N=25\$, it's only around 77%, and for \$N=100\$, only around one third. So:

Abandon breadboard for anything that has complexity. Especially if motors might vibrate, put stress on contacts, or require reliable or high current delivery.