Nobody can help unless you show us your actual ads, your company site etc. There could me a million little signals that are putting people off.
You've also not said what your pipeline looks like - is there nobody coming in at the top? Or are the "great people" not as great as you think? Or the "really deep knowledge of the platform" is in fact less reasonable or badly paid than you think?
If you're stuck, find a good recruiter and pay them properly. They can end-run around whatever the problem is, at least temporarily and give you informed, discreet feedback on how to fix it.
You've also not said what your pipeline looks like - is there nobody coming in at the top? Or are the "great people" not as great as you think? Or the "really deep knowledge of the platform" is in fact less reasonable or badly paid than you think?
If you're stuck, find a good recruiter and pay them properly. They can end-run around whatever the problem is, at least temporarily and give you informed, discreet feedback on how to fix it.