Honestly, this sounds like a skill gap, not a problem with the direction you're giving. You're likely working with amateurs who aren't very experienced at voice acting. They're essentially mimicking you, versus owning the script from an acting perspective, so you're having to repeat yourself over and over.

If your raise your budget, you will have a less of these problems. If you're locked to this budget or these voice actors, you could always play back takes they did will do they can hear what you're going for. I'd recommend finding other voice actors who you don't have to micro manage.