My dad and I are trying to replace a Delta shower cartridge behind the nob. When we take off the cartridge and just let the water flow in from the pipe it has high pressure but as soon as you put in the old or new cartridge the water trickles out of the faucet and doesn't even have enough pressure to reach the shower head.
We have cleaned out those pipes and the blockage happens when you put in the new or old cartridge. The cartridges fit exactly and are actual delta replacement parts. If you need any additional pictures or videos please ask because we are in desperate need.