There's a couple of things going on here, I suspect.
1. If the plastic ratchet is worn, then the indexing gets erratic. Since you're a reasonably-new operator, you may have inadvertently tried to index the turret manually when the ram is at the bottom--and that's a surefire way to damage the ratchet. (Always adjust the turret when the ram is up past the rod twists.)
When I updated my standard turret to a four-die configuration last year, I chewed up a couple of ratchets. Once I got things fine-tuned and developed a good stroke, I haven't had problems for several thousand rounds.
I keep a spare housing and a few extra ratchets on hand.
2. For the index to have changed by that 1/8" or so, you probably have changed your stroke as you've gotten habituated to the press operation. Weight/momentum issues are also in play--a full measure tends to overshoot easier than one only one-third full.
So, check your stroking--I find it typically takes less effort to move the #4 die into place, so that is where I tend to overshoot.
This problem can be fixed by
a. removing the turret,
b. grasping the indexing rod with a padded, parallel-jaw pliers, and using a wrench on the nut on the top to turn the nut a bit clockwise.
Test and adjust again as needed. Note that by turning the nut clockwise, you are "raising" the turret slightly--and that decreases drag and increases smoother operation. (If the nut is really 'low,' try raising the turret by adjusting in 1/4-turn increments--you can easily see if you've gone too high.)
So, persevere--it's part of the learning curve.
Jim H.