Check your alternator output voltage at the points used by the regulator - its case and the output stud.

13.9V is out of range for max output with an adjustable regulator. Locate you losses by moving your test probes toward the battery ONE probe at a time. You may locate much of it between the alternator housing and the engine block - that little cable bears all the output load.

When you're all done, adjust for 14.0-14.5 for winter. You can go down to 13.8-14.3 for hot weather. Remember that an adjustable regulator has NO effect on the max output of the alternator, and is of no use when adding electrical accessories (as some people are led to believe).