Firstly, find an agent you can trust. They are few and far between but they do exist (contrary to popular belief).
Make sure that the marketing of your house is as good as can be.
That should get you viewings. If you arent getting viewings – then that is down to the local market or the price. If other flats are selling around you in reasonable time – then you are probably asking for too much.
Get an agent (that you trust) to carry out viewings for you. Not just their turn-key person who only opens the door. It needs to be some accompanied viewings by someone who is familiar with and incentivised to sell your property. They can get good constructive feedback from viewers during the actual viewing. That will help you build a picture of why it isn’t selling.
Be careful getting other agents’ opinions at this stage. Many of them will just tell you it is priced right just to get you to switch to them – because if they tell you it is over-priced then chances are you will just reduce it with your current agent.
Main thing though – make sure your agent is ticking every box in terms of marketing to get the viewers in.
Good luck!