I have visited Macau on five separate occasions using an American passport:

Arrived Macau 12 October to 14 October
20 December to 23 December
27 January to 31 January
15 February to 12 March
15 March to 9 April

Each visit I was given 30 days of stay. Except the last stay, I was only given 20 days. I left the country each time. Why was I not granted the 30 days on my last visit? --I also had to pay a fine at the immigration office inside the Airport. More importantly, what will happen if I arrive in Macau next time? Will I be allowed in Macau? With a stay of how long?