I already wondered for quite some time why so many of the status changes of municipalities haven't been officially announced in the Royal Gazette, even though the actual administrative act by the Ministry of Interior did already take effect for several years by now. Processing some older board meeting transcripts from 2007, I came across 5/2007 from February 5 2007. This transcript contained both a draft notification (ร่างประกาศกระทรวงมหาดไทย) on the upgrade of Ban Dung to a town (เทศบาลเมืองบ้านดุง), it also contains the drafts of the Royal Gazette announcements for the upgrades of Tha Chang to a town (เทศบาลเมืองท่าช้าง) and of Khlong Chik to subdistrict municipality (เทศบาลตำบลคลองจิก). Whereas Khlong Chik was announced in the Gazette already in July 2007 just half year after the board meeting, the only mention of Tha Chang in the Gazette were the constituencies announced in May 2007.
The only thing which is added to the announcement - apart form the signature of course - is an appendix with the description of the boundary of the municipality including a map. So the only reason I can think of would be problems with the preparation of this appendix, maybe there are still some inaccuracies in the boundary description which need to be cleared before an authoritative map is officially announced.