It works on one try and fails on the next

What it means

This can happen with the WebKit step and with the Relapse kernel exploit. The Relapse README says the browser step may need several attempts, and that the kernel exploit may hang the console or cause a kernel panic.

What to do

1Restart the console and try again

After a hang or a kernel panic, the Relapse README says to reboot before the next attempt.

Source: Relapse-Exploit README ↗

2As a check, keep the same setup and note where each attempt failed

When to stop

! Do not jump to conclusions

This alone does not show that your firmware or your console is faulty. And the fact that the exploit is known to be unstable does not mean every freeze should be ignored.