@Reto *****
This happened to me and the reason was that we stayed in Krabi,I reported later, no problem but still in Thailand, I went to Singapore and when I returned, although I was told by CM immigration there was no need for a new TM30, when the second 90 days came around it was refused with the same advice- go to immigration. It seems that while you remain in Thailand they do not check the TM30, but on returning from abroad they do and in my case the Krabi trip was the last reported TM 30 (by the hotel) CM immigration updated my TM 30 with no fine and now all is well.