Yeah know what you mean there.
I was told by my FI that same thing that the morning slots are the best to go for so booked some 9am slots but these were cancelled due to things such as low cloud or fog which had cleared by 12/1pm'ish.
I really think its just pot luck to be honest. As i said the more lessons i book the more chance i have of flying and have opted for booking lessons at lots of different times of the day.
Cancellations happen, i just accept that now and hope i will get airborne next time.
Runway09