Truffles appear on empty ground. Try to maw some grass (first make sure to fence away some large patches), destroy your pathing and generally, remove debris from farm, as well as items laying around in the open. Pigs need to be petted regularly and have access to food (grass outdoors, hay indoors). The number of truffles per day varies, according to daily luck. Sometimes, truffles appear behind buildings and in such narrow spaces, that you need to search really well for finding them. The profession for Forage that doubles the foraged item, as well as the one which maximizes the quality of foraged items, do actually affect truffles too; so, you might just consider respec.