Some CF cards can be configured to be removable/non removable, PC's won't boot from them when it's set to removable. I don't know if that affects Amiga's.
It could be the cable. I had to replace the one in my A1200 when it started only working sometimes. Also if the cable isn't key'd then make sure you have it plugged the right way round.
But it should be detected in HDToolboox even though it's not bootable?
The cable works, I'm using the same cable as my working IDE HDD.
Is there a chance that this DeLock adapter has any jumpers for Master/Slave?
If it's on Slave it might be your problem there.
That was my initial thought, but haven't find any jumpers, or pin to put one on, on it.
But my trouble just keep adding up. I just installed my new A604 expansion, but the Amiga won't recognize that either

. My 600 simply doesn't want any new friends it seems. Hopefully the ACA620 that's on it's way will be recognized.
Btw, I ordered the preformatted CF drive from Amigakit. I was getting the accelerator anway, plus it is pretty cheap.