Internet per air-stick more expensive?

Dear folks,

till may I was very happy: I put every month 100 Baht on a ais-simcard and got 2 messages: 1 was 'you have topped...' the other 'you have now 30 hours...'

But in June begun the problems I got the message 1 and a message you can not read signes no english o thai and after only a few hours I was driven out of the internet.

I tried also a true-sim put 90 Baht on and I could be little more than 3 hours in the internet.

Any Ideas?

Ciao elzito