Why do my meat burritos not heat well? I've been making freezer burritos and eating them for lunches this week. The first batch I made microwave beautifully: just 45 seconds and they're hot all the way through. They were layered with rice, beans, and corn, nothing else (I don't eat cheese and I wanted them to be non-spicy so I can eat them when I'm feeling unwell). 

The second batch replace the corn with ground beef, cooked and seasoned. The exact same beans, the same rice made the same way. They're smaller, due to using a different package of tortillas; after almost 2 minutes of heating in the same microwave, the meat burrito I had brought still had cold spots in the middle. 

What causes this? Can I avoid it somehow? Should I mix the ingredients instead of layering?
I agree with Brendan's answer above. Having also made freezer burritos ahead of time, I find that it helps if I put some source of re-heatable 'liquid' in the wrap while preparing them (e.g. salsa, not sour cream, for example). It might have something to do with steaming the food instead of directly heating it, maybe? Let me know if that helps.