Do you lose candidates at the 4h assignment stage? If so, that is probably the issue. Otherwise, look elsewhere.
1. Title inflation. Many here are making glib comments about "not actually senior". Great! Call it a "Principal" role and see if that helps. You can always promote any existing Principals to Senior Principal or Staff or God or whatever the new better-than-old-senior title is ;-).
2. Actual inflation. What are you offering? Are you sharing that in a job posting? Hard truth: if you're not FAANG or a handful of other companies, everyone assumes you can't afford them. If you CAN afford them, share numbers in your job posts!
Senior at my FAANG is around $500K at the moment. If you can't beat FAANG, the best way to get people is to offer them more autonomy than they can get at FAANG. But at the Senior level that means making them part of leadership (assuming you're small); Senior at FAANG likely already has more technical autonomy than you can offer. So management and strategy autonomy is a great carrot.
Good luck!