Question: I'm vegan and in general I don't have any problem maintaining my weight, but for the last few months I've wanted to increase my weight and it's been really hard. 

Could others who have had the same problem give me some tips or advice on how to gain weight on a vegan diet?

Answer: 
Oatmeal, bananas, potatoes and soya beans have a high calorie content. Nuts and seeds often have a high fat content, which translates to a high calorific value. If you eat more calories than you use, you gain weight, so these are all good options.

Then there's exercise, muscles are denser than fat, so if you build up your muscles you'll weigh more.
If you want to gain lean weight - instead of a mix between lean weight and fat -, then you need a high protein diet. Researchers don't seem to agree on exact numbers, but, for adults undergoing regular weight training, [most aim at higher than 1g/lb of bodyweight](http://www.bodybuilding.com/fun/maki1.htm) (2.2g/kg for those under the metric system). 

So, not only you need to ingest more calories than your TDEE ([estimate yours here](https://www.supertracker.usda.gov/bwp/index.html)) to keep on gaining weight, but you also need to keep a high protein ratio, as well.

The best sources for piling up protein in a vegan diet are soy protein (usually 20-24g of protein per 30 grams), wheat gluten (70-75g per 100 grams) and spirulina algae (arguably 10-20g per 30 grams). Other sources might include peanut butter, other nuts and beans, but bear in mind most of those sources have low protein ratios. 

Of course, changes in your diet should be discussed with your dietitian. Exercise is also an important factor, since high protein diets have been proven not to provide additional benefits for sedentary adults.