I have had this problem with a number of pages on my site for years, so I agree with tedster than it can be hard to shift.
People search for widgets and google shows the widget makers page. eg. a search for "eggs" would return a page about "chickens"
I thought the most likely cause of the problem was because I had over-optimised the widget page. For example Title, Headings and Internal Links all using "widgets" as well as widget being mentioned on the page more times than is probably healthy.
Unfortunately like you I do not have a lot of control over the pages in question as most of my content is user generated.
I have had a little bit of success in de-optimising the elements of the page I have control over but for many of my categories the widget maker page is still king. Annoyingly a search for "widget makers" usually returns both the widget maker page and the widget page, unfortunately the search volume for that term is only 10% of the volume compared to a search for widgets.