I had a similar issue. Turns out, when I was using SX Installer / File Browser to copy cheats from USB to SD card, it was DOUBLE pasting the titleid!
So, the problem was, e.g.:
SDCARD > sxos > titles > 0100E1400BA9600 > 0100E1400BA9600 [again! a second time!] > cheats > a4cc4c44c07aec14.txt...