OK so you discovered a battery charges fairly slow at 13.4 volts ;)
Now go back and check voltage along the path to discover any big drops at the connection points. Could be any terminal, fuse holder, switch etc. Measure on each side of each connection. Clean, tighten, repair or replace any issues found. Since it used to work OK I think you will find something significant. Don't forget to check ground connections. First suspect is the battery terminals.
For best charging move the converter close to the battery. Connect right on the shunt.
