I'm not being disrespectful but that is one boring looking car. Not that it's your fault of course. It's a lease car, a sales rep car and as someone has pointed out, it's red. Which wouldn't be a problem if it was an M Sport, with 18" wheels as you could change them to black and I think it would fly out but in that colour combo for that model, it looks deeply uninspiring. If it was black, or even silver I don't think you would have a problem. But you are looking for a boring middle aged safe driver who likes sensible fuel economy and low tax and he probably wants it in a low key, non showy colour and red isn't helping.
So I don't think the reason why you haven't sold it, is because it's too cheap. I think it's the wrong colour for that car. For the majority of people of course, not everyone.
You could also try being a little more salesy in your advert.
There will be some one here who will think the way I describe cars is a little bit corny or cheesy or "estate agent" but I think they're wrong so make your own mind up but I find describing what it's like to own the car, rather than "clean bodywork" and "clean interior" inspires the imagination a little. You're a car dealer, you present your cars very nicely, they will know it's clean and can see it's clean. What they want to know is what is it like to drive? Here's a description of a 320i SE Convertible I have up for sale;
"Elite Preowned Vehicles, an award-winning car dealership in Kent, are proud to offer this excellent BMW 320i Convertible for sale. This 2 owner example (previously owned by a Doctor since 2008) drives without fault and has been prepared for retail with no compromise. A superb vehicle, it has covered just 78,000 miles and has just received a new MOT, new brake discs and pads, a new service and an 82 point mechanical health check, including a full scan on the latest diagnostic technology. This car is mechanically faultless. The specification on this car is massive, including full black leather, heated seats, cruise control and Bluetooth. The Sparkling Graphite metallic bodywork is free from any major scratches or scuffs and the excellent 17" alloy wheels are unmarked and shrouded in tyres with plenty of tread. The Dakota Black leather interior is pristine and the carpets are spotless as the entire vehicle has been carefully valeted by a professional car detailer, Wow Factors of Kent.
The 2.0 petrol engine sounds sweet,170 horsepower pulls the car smoothly through the automatic gearbox and the steering is tight and agile. A 3 month warranty tops off what is surely a superb opportunity to own a thoroughly well prepared rare vehicle with excellent aftersales care. Need finance? Let us obtain a fantastic deal for you through our prime finance lenders, even if you have little or no deposit. Buy with confidence from an award winning 5 star rated Autotrader dealer."
Make your own mind up of course, I hope the above helps.