One of the reasons that alternators replaced dynamos in the automotive industry is that the latter can't tolerate high rotational speeds and so is geared to run at speeds similar to the engine. Cut-in speed on a dynamo is usually around 1000 rpm so there is no charging below that speed. An alternator can, by its design and construction, safely rotate faster than a dynamo so it is geared accordingly and is generating current even at engine idle. This, and the fact that an alternator is more efficient, encouraged Chrysler to introduce alternators in the early 1960s to cope with the increasing electrical demands of cars at that time.
In short, your alternator was probably never designed to charge at idle engine speeds. It's cut-in is probably above that speed!
When my voltage regulator failed due to corrosion, I converted to negative earth and installed a Lucas alternator. It's one of the best changes I've made to the car. I have no hesitation in making changes to improve reliability if it doesn't change the characteristics of the driving experience hence, the alternator, tandem master brake cylinder and electric fuel pump,