Cream cheese icing dilema!

I work at a small cafe and bake some baked goods for them. I just graduated from culinary school and feel pretty experienced with cakes and such, but this is the first time i've ran into this problem. I often make my buttercream icing with a sugar and egg white meringue. At work i make it with the meringue and then add butter and vanilla. Really simple and easy. I've never had a problem with it. Recently i made carrot cake and wanted to do cream cheese icing. In a book i have they use the same exact icing recipe and just add cream cheese in addition to everything else. So i did that. But my icing would not come together. No matter how long i mixed it, it was still separated looking. Normally if you keep mixing it, it comes together correctly and i figured it would now. I even chilled it then rewhipped it and tried whisking small batches by hand and nothing seems to bring it together! I got so frustrated i just put it in a container in the fridge and made just plain buttercream. But i don't want to waste the icing! So does anyone know how i can fix it! Thanks!