I have a hard time gaining weight so my Dr said it was ok to have a daily ice cream for the calories. She also told me to add more protein so added 2 peanut butter sandwiches a day and a protein shake (like Boost or a homemade one). Now I'm on track for normal weight gain :)
Are you eating enough? With my job I figured or the only way I'd gain is if I ate a lot more than the normal extra 300 calories. Try tracking your amount of exercise and nurturing and remember add long as you are taking your vitamins the baby is getting the nutrients it needs. Babies have tiny bellies and get fed first.
I lost 15 lbs before I finally started gaining weight with this pregnancy. It's not that uncommon. If you're doctor isn't worried, you shouldn't be either.
Are you overweight now or overweight before you became pregnant?