Your water heater was working on propane before you filled the tank, right?
If so, it is likely the valve on your propane tank was opened too quickly causing the safety mechanism to limit the flow. This happens a lot!
Shut off the propane at the tank valve.
Turn on a stove burner to drop the pressure in the propane line then turn the burner off.
Now very SLOWLY open the valve on the propane tank.
To purge air out of the propane line, open a stove burner again for a few seconds then close it.
That will take care of it if the safety valve was causing your problem.
Let us know.
Sherm & Terry w/rescue Eydie (min Schnauzer) & Charley (std Poodle)
2015 Jay Flight 27RLS, GY Endurance (E), Days: 102 '15, 90 '16, 80 '17, 161 '18
2006 Ford F350 PSD, 4WD, CC, LB, SRW, Cmpr. pkg, Equal-i-zer 4-p, Continental ContiTrac TR