On tub/shower valves the outlet for the shower is smaller than the outlet for the tub. The larger demand needs to be hooked to the tub outlet. You can try removing the water restrictors but it violates federal law if you do. There is no reward for turning you in but…..
Shower heads talk in gallons per minute (GPM). We used to have 10 to 15 gpm but now are limited to 2.5 gpm with the restrictors. Try removing the regular shower head to see if the water still comes out the rain head. This is the same problem as when people have water coming out the shower head when they only want it to come out the tub spout. Many of us are turning the tub/shower valves upside down when we install the rain heads.
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