Firstly, do you know the date of the dissertation? Dissertations are submitted each October and added to the library the following spring. A few students apply for an embargo, which means there is a further delay of one or two years until their dissertation is available.
Secondly, please check your spelling and wording. Try using fewer words from the title, and/or omitting the author's name.
Next, could this be a dissertation from the New York campus? If so, check that you are searching without limiting your results to London alone. This is easy in Summon, where by default both the London and New York libraries are searched together.
Finally, since 2008 the London library has only kept dissertations awarded merit or distinction. Faculty sometimes provide lists of dissertations including those that did not do so well, to indicate the range of subjects you might consider for your own dissertation.
Of course, ask the librarians if you're still not sure.
