[...]they were mobs my level so they should have dropped SOMETHING.
Did you look up how drop rate works?
Furthermore, the chance of no item drop for 1000 monsters is greater 0 (so unlikely but possible) even if the droprate of an item is 99%.
But spoiler, most drop chances are between 3% and 10%. So assuming your luck potion doubles the rate, you end up with 6%.
But drop rate is a very very complex topic that is affected by, for example, if you are in a group or not.