My goodness, plenty - sometimes the fault of the advertising people, sometimes actually the advertiser's fault - and sometimes because of other factors.

Incorrect brand positioning. It goes without saying that if an advertiser doesn't fully understand the true nature, character, perception and positioning of their brand, the finest advertising available won't sell it (wrong strategy, wrong voice, wrong stage etc).

Ignorance of Competitors & The Market. It really is essential to have as much information and knowledge as possible (statistical, historical and research) on both the industry and your competitors. If you don't know the playing field or your opponents - you're going to lose!

Incorrect Target Audience. I remember a client who had had reasonable success from his regional press advertising for many years and always presumed that his brand was far more 'upper end' than we did. My media chum then ran a MOZAIC post code analysis (demographic profiling) on the client's existing customers which showed a rather different story. The client had probably been wasting 30% of his advertising spend!

Incorrect Media selection. It's absolutely pointless giving a lecture in the 3rd floor Conference Room if everyone's sitting up on the 4th floor, yet some advertising can be strangely similar. You could be talking to an empty audience!
Media Planners now have many powerful tools at their disposal to help them obtain highly efficient media selection, but it can still go wrong. And when it does it's literally a waste of space, or time...and money!

Poor Creative work. If all the correct information has been made available there really is no excuse for poor creative work, and that means incorrect strategy, poor copy, ugly typography & aesthetics, bad reproduction or perhaps simply little understanding of how the specific type of advertising should be structured (see The Nature of the Beast).

Influence of other factors. Even if all the right boxes have supposedly been ticked (sorry, see 'Bullshit Bingo') there are still many other external factors that can affect the success of your advertising, such as seasonality (try selling lawn mowers in September), the weather, (or wallpaper on a hot Summer weekend), the current financial environment, impending technology, stronger competitor's campaigns, shift in market trends. And often these factors aren't obvious.

Problems with the advertiser. Oddly, the problem could lay closer to home. Is the product at fault? Are the Sales Team doing their job? (I can think of many instances where this was the case). Is the 'call to action' system efficient?

What if I just don't advertise at all? Usually not a good idea.
If you don't advertise no-one knows you're there, or worse still, they'll create their own perception of you or your brand ("tried it years ago - it was terrible!").

All these points may be contributory factors to the success or failure of an advertising campaign and it is just as important to know exactly why it was successful as to why it wasn't.
The reasons may not be what you assume.

The only way to understand the impact of advertising is with...

REGULAR RESEARCH, TESTING AND RIGOROUS ANALYSIS.