Change your estate agent and quiz any new one about their experience in selling retirement flats. There may also be a local estate agent that specialises in this type of property. There is in my locality.
We had to sell a small bungalow some years ago, not a sheltered bungalow but one on a small estate where most owners were retired. Our first agent was useless, they overpriced it, which I queried, and then when it didn't sell blamed us for insisting on a high price. We changed agents, dropped the price and it sold quite quickly.