creating the hole for the sensor

[SDJeff]

Sometimes, I find drilling into concrete a a bit of a challenge depending in the slab; some slabs seem harder than others and control of the bit is more difficult. QUESTION: What factors make a hole unsuitable for testing? I've had cases where the depth was right but the sensor would not go in even using the tools and recommended methods; then, after one more (in and out) with the bit, the the sensor went in easily without much effort. If the sensor goes in too eaisly is the hole still usable?