if a property remains unsold after three months is there any point in changing agents? I would guess that anyone who is looking would already be familiar with whats on offer through Rightmove. but what do they do if they are not looking in that area? Do agents collude with each other by sending details to other agents outside the area (and sharing the fee).
It may be worth changing agents depending on the property you are trying to sell. Some Agents get lots of lower end starter homes on their books and will have people on mailing lists waiting for that sort of property, some agents have more rural properties and different clients. Make sure your property 'fits' in with the other houses the Agent is selling -if not do a search on line of the particular type of house you have for sale and see which Agents have sold the most.
If a property I was selling remained unsold for 3 months I would be looking very closely at the price.
Agents don't have to collude, they can all see each other's sales details on Rightmove.