A very common problem, and one that I actually have, is that flexion during the front squat and clean will result in the weight being tipped forwards and you can drop the weight.
In my case, it is simply weakness - I need to train more at keeping good form- I did start working on it but, as always, things are a work in progress - read about how I increased my front squat by 12% in less than 2 months...
For high level athletes, weakness is not often the problem. Often it is simply a coordination issue or their abs are too tight/overactive causing the spine to flex. Learning where and when it occurs is important. You can then teach the athlete to train a different pattern and they smash through that plateau.
Here is a video of Vaughn Langtry, Australian Regionals Competitor from CrossFit BodiComplete and his front squat analysis I did in Jan 2014. He was able to break through his plateau in the clean after a few days of training his low thoracic stability and releasing his upper abdominals.
Give it a go and let me know how it went!