There may be a few reasons. Pudding can be a finicky thing; even stirring your pudding too vigorously can make your pudding runny. More likely, though, if your pudding isn't brought to a full boil, it won't set correctly and may be runny. If you did bring it to a boil and it's still runny, then it just needs to cook more. if it's still runny after cooking the allotted time, you may have added too much liquid.