New delta shower valve barley any water coming out of shower head can take valve out and turn back on and have water tried two different valves same thing from both

I will assume this is a single handle. When you took out the cartridge, did you open the valves and flush it out? What kind of piping, PEX, Copper or CPVC? Unscrew the shower head and turn the water back on. Did you make sure the small ball valves under the escutcheon plate are fully open?