Is it Monterrey Jack, or Mexican soft white cheese? You know, the crumbly kind that looks like Feta? That could be the problem? I have never, ever seen it in this neck of the woods. I can't even remember what it is called, but lord I remember how it tastes....
I think you can be flexible with exactly which white cheese you use. When I heat the Monterey Jack with sour cream over my makeshift double boiler, it certainly looks and feels right. It's just the spicy element that I'm missing somehow. I've used jalapenos and the green chiles you can buy.
Back in the US, I tried it with Asadero, which I think is supposed to be more authentic, but that tastes just like Monterey Jack to me.
One suggestion for easy mexican enchiladas is to use the enchilada kits sold at Tesco or Asda to season/cook your meat (I use chicken.) I think make my own Asado sauce by heating Cream of Tomato soup from a can, with jalapenos, black pepper to taste, a little fresh garlic, and some diced onions. Once the onions have cooked, I dip my tortilla in the sauce to moisten, roll up my enchilada using my chicken filling w/cheddar cheese. I arrange my enchiladas in a neat little row. Pour the rest of the Asado sauce on . Pour on the "white cheese sauce" (mentioned above) and then bake until I think it's done. It really does taste like something you'd order at a restaurant, and it satisfies my craving as much as it will ever be satisfied in this neck of the woods.