I attempted a mirror glaze for the first time and everything seemed to go well until it came to the pouring part. My buttercream crumb coated cake was very well chilled but the glaze just ran right off and only left a very thin coating.

anyone have any suggestions as to why that happened?

i chilled the glaze and then rewarded it in the microwave . It was about 30c, when I poured it I was able to put my finger in comfortably.

also is it necessary to chill it or can you just leave it on the counter till it cools down?

thanks

without knowing the recipe you used it's difficult to say. I learned that the buttercreamed cake should be very cold & miroir at room temp. 30c is very warm. 

Your glaze was likely too hot. Also, microwaved glaze may not be the uniform temperature throughout. I have better results with using freshly prepared glaze which has cooled down sufficiently, then with refrigerating and then re-warming the glaze.

Thanks, I will try again without refrigerating the glaze