Steve
What's your control like, are you regularly running high BG's? As this can keep weight down..
Something two many highs or generally running a tight control range running above normal will keep weight lower, preventing weight going on..
I'm the same with boat, I'm a stone lighter than pre-diagnoses, and I've never managed to regain all my weight, and unfortunately for me, it's nothing to do with control, even after over 4 years on a insulin pump which has enabled me to get even a tighter control range than I had with MDI, I still have put any weight on, so 23 years down the line, I've given up and just except, but it's not all bad as well I am female..
But as Nigel says, try upping your calories intake see if this works..
But you will have to watch what you up the intake with..
Extra Carbs could destabilise your control, by making the insulin adsorption rate fluctuate..
Protein, could mean that you might have to increase insulin a little to counter act it, as some protein breaks down into a carb impacting on blood glucose.
Fat.. Could effect either you food adsorption rate slowing it down, which can destabilise, so may need to think about injection timing to counter react and/or it can increase your insulin resistance so might again need to increase insulin ratio's slightly, again same as carbs could cause the the adsorption rate to fluctuate
So which ever way you chose it's wise to increase your blood glucose testing, and note down the effects on control..