I too had a similar issue to you in a previous month. I started play on the promotional day in question at 11pm GMT. I had a number of Blackjacks and assumed I would receive a bonus on my account. I did not receive one, and so queried it with CS. They told me all 1500 BJs were hit in the first few minutes. I do find this surprising.
Assuming a
BJ rate of 0.0483 per hand and the player plays 10 hands per minute - notice these are very liberal estimates - 622 players would need to be playing
BJ at the start of the happy hour for 1500 BJs to be won in the first five minutes. I am fairly certain I got some BJs in the first five minutes.
I played at ITV Casino, though I think all the Partycasino Casinos would be involved.