Hi!
Are you using the FreePBX distro and if so which version and what hardware are you using?
Since a recent update my FreePBX 10.13.66-6 DAHDI extensions no longer work after a reboot.
I have a Sangoma A200 and someone else with an OpenVox card has a similar problem, see DAHDI Errors - Permission denied
Good luck and have a nice day!
Nick