Seti has been limiting caches server-side for quite a while now, supposedly due to DB congestion.
No matter what your settings are, you are limited to 100 CPU tasks and 100 GPU tasks in cache.
That's 100 no matter how many CPU cores, and 100 no matter how many GPUs you have.
That is why you cannot build a sustainable cache anymore.
There is likely no problem on your end. If you check your Boinc messages, you will probably see some that say 'this computer has reached a limit on tasks in progress.'
If or when this will be changed, I have not a clue.