Probability of getting an item by mining

I wanted to ask, how the probability of getting an item, after mining/farming for it (ex. mining gold) is being calculated? It seems to me that I quite often don't get an item, even though I meet all the requirements. (Mining lvl 64, T7 Pickaxe while mining for gold)

Or am I doing something wrong here, or is it simply random?
Thanks for the help!

 

is your tool broken?

other than that-
you have a percentage chance of gaining a resource from mining a node per bracket of 10 levels. by the time you're at 30 mining you will have the maximum chance to get a material from a level 20 resource, and so on

out of that there's an 89% chance to get a tier 1 ingot, 10% form tier 2 and 1% for tier 3.

 

Also if you are carrying more than one tool of the same profession, your chances of obtaining a material is reduced.