Dahdi drivers won't install

I’m using the full FreePBX 14/Linux 7.5 distro on a new machine and the AEX410 drivers won’t load. FreePBX suggested using the manual DAHDI installation method, but that fails during the initial “make” process because the full kernel source files are not found. I followed the procedures here https://wiki.centos.org/HowTos/I_need_the_Kernel_Source but the error persists. I’m not sure where to go with this problem. I’ve confirmed the kernel that’s running matches the source files downloaded letter by letter and number by number. Any suggestions?