Hi Charlotte,
You may be using up some body fat for energy after your heavy lifting sessions still, so that may be part of the reason the scale isn't moving. And if you are lifting that heavy to failure all the time, you know you are burning some major calories!
When I used to do home health, we recommended a high protein shake or snack before bed to help with weight gain. We also encouraged Ensure or Boost drinks as well; these have quite a few calories as I remember. You know your metabolism slows as your sleep, and your body can better absorb the protein and nutrients to help with muscle growth/weight gain. If you use one with lower carbs/sugar, you really won't need to worry about putting on fat. You could probably afford to add some peanut butter to a shake as well; the added calories would help and the fat is pretty healthy.
Another thing we would say is to add healthy calories to regular foods: for example, non-fat powdered milk can be added to sauces, puddings, a glass of skim milk. Throw a bunch of cooked egg whites onto your salads or into your oatmeal. Use a little more olive or canola oil when you saute or stir fry. Add an extra serving of nuts to your snack, cereal, veggie or fruit salad.
HTH,
Julie