I am on a LCHF diet and I initially (9months ago) found it difficult to sustain weight, and initially lost a stone, but I have put it back on again and a little more. I have done this through eating more fat, choosing fatty cuts of meat (lamb chops etc), avoiding chicken breast (little fat), eating lots of butter (a pack per week) eating lots of double cream (small pot per day) and 30g coconut oil per day. A buletproof coffe with 30g coconut oil and 30g butter has around 500 calories, this very much helps maintain weight. Double cream with, for example omlets, heated and mixed with grated cheese over veggies (cheese sauce). I am sure you will be able to put some weight on if you go 'higher' fat.
Dr and dietician both amazed at how good my cholesterol levels are, better than a year ago before LCHF. My carbs intake is are around 36g per day. Mainly through salad and veg. No bread, pasta, rice or potatoes. I am 12 3/4 stone... 6'-2".
Edited to add... Diabetic dietician said my high fat diet and blood results (for cholestrial, kidney, liver etc) go completely against all her NHS training, but that her colleague, a T1 diabetic nurse, is also on a LCHF diet, and he has the same results as I do.... So she told me to carry on with my diet and change nothing as it is obviously working for me.