Try small meals throughout the day. If you're getting full faster, it could be you just don't have as much room with baby getting big. I haven't gained any weight yet (18 weeks) although I am gaining size, or rather my weight is moving to my belly. I think because I'm eating better than I was before I'm gaining and losing weight at an equal rate. That could be what's happening with you.
Maybe the trick isn't eating more than you do, but eating foods higher in carbohydrate and/or calories. Maybe try a nutrition shake with a meal or 2 each day.