Tiamet, I noticed the same thing with dilating way back in the day. For me it took about 3 months post-op to able to use the largest dilator at my original depth.
Here's the thing -- you aren't losing depth. You're still at 5" with the purple. It's just that your width deeper inside hasn't grown yet. Think of it this way -- your vaginal canal isn't cylindrical at this point, it's
tapered, so the wider dilators have get to reach bottom. Keep plugging away, it takes time to create more space and to keep stretching out the skin.
I found I had to press down enough that it started to hurt deep inside to gain deeper width with the larger dilators. With the more comfortable one, I'd rotate the dilator, applying pressure opposite the direction the curve points toward, effectively using the muscle of the introitus as a lever. It also helped to, um, slide it in and out repeatedly in a motion resembling vigorous coitus, all the way down to the bottom and then some, each time... while pleasuring myself (so as to release endorphins that ameliorate the pain, of course).
I also found that by going back to the thinner dilators, and bearing down, I could slowly --
slowly -- start building more (tapered) depth. This is very slow going -- like 1/2 inch in another six months, if not longer. (And this is not, admittedly, a very pleasant exercise. Ouch.)
Finally, I haven't located any dilators that are bigger than the orange, but there are certainly sex toys on the market with more girth when you're finally ready for that -- although frankly the real deal will probably be more effective at that point

.