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.
I would like to answer this question both for vegetarians and vegans from the Ayurvedic viewpoint (the only difference being vegans do not take dairy).

Ayurveda is a system of well-being and medicine that originates in India, and it recommends a vegetarian diet. According to Ayurveda, one can gain weight by eating *vegetarian food that is grounding, warm, oily, smooth, stabilizing, and substantive.* Other factors like a regular daily schedule and a light exercise routine are also involved.

It is recommened to eat three heavy meals at the same time everyday.

You can read more at this [article](https://www.banyanbotanicals.com/info/ayurvedic-living/living-ayurveda/health-guides/weight-management-guide/an-ayurvedic-approach-to-gaining-weight/):

> 
> ...You will want to orient your diet around healthy, whole foods that are
>  deeply nutritious. Choose foods that are warm, moist, oily, grounding,
>  nourishing, smooth, and stabilizing, over foods that are cold, dry,
>  light, or rough. Favor the sweet, sour, and salty tastes over the
>  pungent, bitter, and astringent tastes. Build your meals around staple
>  foods that naturally taste sweet, such as fruits, grains, root
>  vegetables, milk, ghee, fresh yogurt, eggs, nuts, seeds, and
>  vata-pacifying meats. Nourishing and rejuvenating dishes such as Sweet
>  Potato Halva and Urad Dal (Split Black Gram) are fantastic, as are substantive soups and
>  stews. You can also include grounding, nutritive snacks in between
>  meals. For instance, fill a jar with dates, pour warm
>  Ghee (clarified butter) over them and let the ghee solidify. Eat one or two of these ghee
>  dates in the early morning or any time between meals for a
>  rejuvenative boost of energy. Other supportive snacks include soaked
>  and peeled almonds, soaked cashews, and date shakes.
>  Experiment with what works for you and keep snacks handy so that you
>  can reach for quality nourishment rather than a quick fix of empty
>  energy (i.e., sugar or junk food).
> 
> 
> 

This [page](https://www.banyanbotanicals.com/info/ayurvedic-living/living-ayurveda/diet/vata-pacifying-foods) gives a list of fruit, vegetables, grains, legumes etc to be preferred and avoided for weight gain.