What is the BTU input rate and EF of your new WH? What was the size, BTU input rate, and EF of your old WH? What is the shower flow rate?
If inlet water is 40F and outlet is 120F and HW flow is 3 gpm, you need 2000 BTUs/minute input to keep up. A WH with 50,000 BTU/hour input with 20% flue loss only adds 667 BTUs/minute.