I see a couple things that could be going on:
1) Sometimes when they get good #s (like the blues you saw a couple days ago), their body responds in protest, and they throw high #s for a while. This happened a lot with Bix, and it took a while in fairly steady blue #s before he settled down, and then after that I was able to reduce his dose. I think that is what they call "liver training".
2) Sometimes with PZI, they will respond well on the first cycle at a new dose, and then the good response disappears. I found this really confusing, but basically the bottom line is that it means the dose isn't enough. It's like their body integrates it and you don't see the effect that you first saw, when it was a shock to them at first.
I can't say for sure that either of those factors are the definitive explanation, just trying to help you understand what might be going on.
It looks like you are getting a better drop today. If it were me, I'd probably try to get in a +6 or +7 test, and then if the PMPS is a lot lower than this mornings, I'd probably stay with this dose another cycle or two and see if the improvement continues. If the +6 or so isn't much lower than the +4 and the PMPS is back up fairly high, I'd probably go ahead and raise by 0.2u.
One way to look at PZI is to calculate the % drop from PS to nadir. Something in the 50% - 65% range is ideal IMHO. Steeper than that you are risking rebound, shallower than that and the #s probably won't come down enough. Today's looks to me like a good drop, so if you are lucky the PS tonight might be a lot better than what you have seen the past day or so.