She's only 6 months! At 6 months babies don't eat much!
At 6 months though you can make food more interesting and calorie filled (think butter, cheese, oil, meat, carbs etc.) You may want to introduce things slowly but don't just stick with fruit and veg purees because they aren't full of calories. Or at least make it carrot and swede puree with butter, cauliflower cheese, pureed roasted veg (roasted in olive oil), fish and broccoli puree with mash, milk and cheese, beef with sweet potato etc, etc.