Replacing meals with weight gainers/protein shakes?

Is it okay to replace meals with weight gainers? I'm a female and I'm looking to build muscle. I drink protein shakes and weight gainers with milk. I also eat, but not very much. For some reason, I have a hard time getting enough calories down. I keep track of my cals and for example today, I'm under about 1,200 calories! And it's already 8pm. So I made a weight gainer shake to reach my calories goal...the shake has around 1,260 calories. Anyways, is this okay? I know I should EAT most of my calories but usually I can't. Am I missing out on important nutrients?
This weigh gainer has vitamins and minerals as well as protein.
-Thanks a lot