My nieces who are a month older than Oliver, had already turned over by their 3rd month. Yet Oliver was about to complete his 3rd month and move on to his 4th month. But he had not turned over?

Was it the comfort and ease of a mattress ? 

Once I placed him on the HARD floor, within a week we observed him making efforts to turn and by the second week, we saw him turning over.

Why loving Heavenly Father allows us to go through – HARDships?

It is because He wants us to progress from one stage to another, from glory to glory & to press on to Spiritual maturity!!