For me personally, the biggest ways that I find I am not compliant (when I'm not) are:
1. Not enough non-starchy veggies. This is an ongoing struggle, even when I am really good about following the plan. I am quite capable of going through the day on only oats, potatoes, beans (with some tomato there) and maybe a piece of fruit.
2. Too much sugar. Again, an ongoing struggle.
3. Too much vegan convenience food. I go through phases with this stuff and it's not that hard for me to eliminate - if I make up my mind to do it.
4. Flour, especially white flour. Phases.
Risk factors for me are:
1. Getting very hungry. I then go on eating binges in compensation.
2. Being really good during a time of temptation (parties, restaurants, etc). Afterward, I feel like I "deserve" a treat for being so good and that it's normal to have treats and then sometimes I find myself not being able to stop having the vegan treats.
I can't think too much about what to eat because invariably there's something not as healthy that sounds more appealing than the healthy option (for all kinds of reasons). It's better for me to quickly make something than to think too long.
It is hard to eat at other people's houses - especially when they have tried really hard to make something for you and it's not quite right. Like they've cooked vegan, not no added fat vegan (as wordweaver said). I do not know how to refuse this or even if it is best to refuse this or just eat a small portion and move on.
I see that many people (myself included) look at their diet and say that because it's vegan, it's healthy - and that is certainly not the case. Most "vegan" people I come into contact with (this board excepted, of course...) have pretty poor diets from a nutritional standpoint. Just because something didn't come from an animal doesn't necessarily mean it's healthy. I've eaten vegan stuff that was so full of oil and processed junk that it would have been less health-damaging to just eat a piece of roast chicken (which I probably would have enjoyed more anyway).
I've noticed on this board that sometimes people feel they are following McDougall and it's not working for them when they:
1. Eat a lot of tofu, TVP and other meat/dairy substitutes (a lot meaning more than once or twice a week).
2. Use flaxseed oil.
3. Eat nuts/seeds daily (assuming there are remaining health problems).
4. Use a lot of soy or other nondairy milks.
5. Base their meals around green/yellow veggies and not a starch.
6. Lots of fruit juices or even fruit smoothies (not as bad as juices, but still not supposed to be an everyday item).
7. Too many flour products. Although they're allowed in the regular program (in the whole grain form), I don't think they're meant to play as large a role as the starchy veggies like potatoes or whole grains like rice.