Switching regulator efficiency TI LM2678 - measured vs. datasheet
Things that can likely go wrong: -


Building it on breadboard - contact resistance versus solder
Building it on breadbaord - contacts/track volt drops
The inductor gives no clue to how it will perform at 260 kHz - choose one that does
33uH is probably not the best choice - 47uH is a better choice for 20V in and up to 2A out (assumed LM2678-12 and adjustable version)
Bad positioning of input and output capacitors
Bad choice of input and output capacitors (electrolytics are quite possibly a bad choice so follow the data sheet as to its recommendations - it has many)
Inductor leads too long
Measurement errors in meters
Measurement errors in probing
Schottky diode of correct type not used (DS says: From Table 10 a 5A or more Schottky diode must be selected)


Breadboard build is the worst culprit. Trust that it will work, lay out a PCB, come back here and get someone to review the layout. Don't build switchers on BB.