More info on the blue bulb might help to see if it's in a good spectrum.
Contrary to popular opinion on light color blue is a plant spectrum(410-455nm) and
from what I have read on here it is used by the plant in pigment production and
flowering. Thus it balances the red which is used for growth.
But I should first say that I haven't done high tech so my info is from what I've read
both directly about it and what solves problems people have/w it.
In other words wait till it's co-signed by others before you act on any of it.
Opinion: Your light is higher than your CO2 level can accommodate.
Before the CO2 comes on if you check it then and towards the end of the "on" period of it, you should get a one point drop. 7.5 to 6.5 for example. This indicates that maximum or very close to it has been absorbed by the water.
This is not consistent/w what you have reported in your tank. So I'm saying the light is too intense for that CO2 level and hindering growth.
I still find that four bulbs(depending on how good the reflectors are) for a tank 18" tall
is overkill. Scroll the page on this link. Check to see that you are using enough ferts.
Milwaukee regulators are not a preferred brand very much from what I read on here.
But you might try to increase the amount you are using till you get that point one drop
and see if that helps your problem. If you can't raise it some you may need to cut the light to balance the system.
So too much light/not enough CO2 and possibly not enough ferts.
Just a system that needs a little more time for you to tweek it better.