As a layman (please humour me, I realise this could be a daft question) is failure to report results generally a sign of anything relating to the outcome, or is it equally likely to be something mundane?
Not a daft question at all. There're probably a range of factors.
From Ben Goldacre's paper[0] pharma industry sponsors are more likely to be compliant than non-industry sponsors, and sponsors of large trials more likely to be compliant than sponsors of small trials. This may suggest that it's partly a matter of human resource to actually compile the data and report it onto the public websites in question. Here, I'd also consider Hanlon's razor - busy people with other priorities, rather than nefarious hiding of important study data.
One point to recognise is these analyses (there are several sub-sites running [1]) explore reporting of the study results onto specific public-domain databases, per FDAAA (or other) requirements. However, it's possible that results may be reported via other routes (poster or presentations at congresses, or publications in journals) - and this might be driven more by positive results than negative. I don't think the study methodology would catch examples which had been publicly reported, but via the wrong route. (These cases would still obviously be non-compliant, but maybe less of a shade of grey?)
Also as a layman, my understanding is that simply having only "interesting" results skews the conclusions that can be drawn by meta-analyses. When you look at a bunch of studies and try to learn something from them, you need the inconclusive and negative ones to be included, in order to learn valid things.
By omitting inconclusive or negative results from publication, the whole of science is misled.
In addition to the reasons listed by other commenters, my feeling that it is also just a matter of the trial results being delayed.
This can happen for a number of reasons.
* Insufficient patients are not willing to consent to enroll in the trial. This is usually solved simply by extending the duration of the study
* Doctors are to busy to obtain informed consent from patients, and take the easier path of not mentioning that they qualify for a trial. This happens often in hospitals that join a trial but are not directly research oriented.
* Delay in the data-collection, statistical analysis or interpretation of the results. As in other fields, statisticians are constantly kept busy, and often change profession. Lack of personnel can delay the result of the trial.
