You aren't the 1st one to report it.
I unlocked my 555 and got quad core Denab B55
with the 6mb cache.
Seems to be down to luck, if you get it you get it, at least you have 4 cores, lots don't get that.
Uhh... That's because the 555 has a deneb core and already has the cache...
Your Athlon does not have the cache present at all, so it cannot be unlocked. Only very early Athlon IIx4 could be unlocked with the cache because they were actually deneb cores with disabled cache. This is no longer the case and newer Athlons do not have the cache at all.
If you have a C2 stepping Athlon IIx4 that also reads as a deneb core, I believe that is the only case where you can unlock the cache. I have an Athlon IIx4 that does not read as a deneb core. There is nothing to unlock. I tired anyway. There was nothing.