New shower reduced pressure-GrohFlex

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Forgive my novice here, I'm a homeowner of a house with some strange plumbing issues so have used this forum for advice several times in the past, but wanted to ask the experts.

Just had my bathroom shower remodeled (handheld and showerhead, no tub). Had no issues with water pressure in the past. We used the GrohFlex universal rough in with Grohe fixtures. Now the showerhead has much less pressure than the handheld. It's not the hardware fixture itself since when you plug the handheld shower into the showerhead outlet the pressure is much less than at the handheld outlet. The handheld has great pressure, no issues there. Took the showerhead off and let it run in case there was any debris in the line. Didn't solve the issue.

Plumber came back and said he didn't see anything wrong, but my bathroom supply company contacted Grohe for a new GrohFlex cartridge, which arrived yesterday. Interestingly, Grohe had 2 questions for me, which didn't make sense relative to the straightforward 90 and 180 degree install.
  1. Did he pipe the shower head out of the bottom port? There will be more pressure at the 4 o’clock position.
  2. What position is the head going on at?
I don't see anything on the instructional diagram with a 4 o'clock position or options for how the head goes on.

Any thoughts on what could be wrong? Use a different plumber unaffiliated with the remodeling company? Of course the shower is totally finished.

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New England
Is the upper head in your shower a rain shower head? If so, by design, there's little outlet pressure...the water comes in, gets dived up and falls out. In a typical shower head, the nozzles create a restriction, and that (Bernoulli principle) causes the water to speed's not the volume necessarily, it is likely the velocity dictated by the nozzle design.

Some diverter valves have a built-in restriction when they are designed as a tub/shower valve and you use shower heads on both outlets...the outlet to the tub is larger, the one designed for a shower head will often have a restriction in it. At the head, if what's coming in is not more than what can get out, the flow won't accelerate through the nozzles.
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